Optical filter, and analytical instrument and optical apparatus using the optical filter

ABSTRACT

An optical filter includes a first substrate, a second substrate, a first reflecting film disposed on a first opposed surface of the first substrate, a second reflecting film disposed on a second opposed surface of the second substrate, a first electrode provided to the first substrate at a peripheral position of the first reflecting film in a plan view, and a second electrode provided to the second substrate and opposed to the first electrode. At least one of the first opposed surface and the second opposed surface is provided with a step section, and an initial gap between the first reflecting film and the second reflecting film is smaller than an initial gap between the first electrode and the second electrode, to thereby allow control of the gap between the reflecting films with good accuracy.

This application claims priority to Japanese Patent Application No.2010-058301 filed Mar. 15, 2010 which is hereby expressly incorporatedby reference herein in its entirety.

BACKGROUND

1. Technical Field

The present invention relates to an optical filter, an analyticalinstrument, an optical apparatus, and so on using the optical filter.

2. Related Art

An interference filter has been proposed that has a variabletransmission wavelength (see JP-A-11-142752). As shown in FIG. 3 ofJP-A-11-142752, a pair of substrates are held parallel to each other, apair of multilayer films (reflecting films) are formed on the pair ofsubstrates so as to be opposed to each other with a gap having aconstant distance therebetween, and a pair of electrostatic driveelectrodes are provided for controlling the gap. Such a variablewavelength interference filter generates electrostatic attractive forcein accordance with a voltage applied to the electrostatic driveelectrodes to control the gap, thereby making it possible to vary thecenter wavelength of a transmission light beam.

A requirement for such variable wavelength interference filters is tocontrol the gap between the pair of reflecting films with good accuracyusing an electrostatic actuator. Since the target is the wavelength oflight, nanometer gap accuracy is required. In particular, a variablewavelength filter that makes it possible to perform wavelength selectionin a wide band requires highly accurate gap control with a finedisplacement within a limited drive voltage range while achieving largegap displacement (movable range).

SUMMARY

An advantage of some aspects of the invention is to provide an opticalfilter, an analytical instrument and an optical apparatus using theoptical filter each capable of controlling the gap between the pair ofreflecting films with accuracy using an electrostatic actuator.

According to an aspect of the invention, there is provided an opticalfilter including a first substrate, a second substrate opposed to thefirst substrate, a first reflecting film disposed on a first opposedsurface of the first substrate, the first opposed surface being opposedto the second substrate, a second reflecting film disposed on a secondopposed surface of the second substrate, the second opposed surfacebeing opposed to the first substrate, and the second reflecting filmbeing opposed to the first reflecting film, a first electrode disposedon the first opposed surface of the first substrate at a peripheralposition of the first reflecting film in a plan view, and a secondelectrode disposed on the second opposed surface of the secondsubstrate, and opposed to the first electrode, wherein at least one ofthe first opposed surface and the second opposed surface is providedwith a step section, and an initial gap between the first reflectingfilm and the second reflecting film is formed so as to be smaller thanan initial gap between the first electrode and the second electrode.

According to this aspect of the invention, the initial gap between thefirst reflecting film and the second reflecting film is formed so as tobe smaller than the initial gap between the first electrode and thesecond electrode. Here, the electrostatic attractive force F can beexpressed as follows.

F=(½)∈(V/G)² S  (1)

In Formula 1, ∈ denotes the dielectric constant, V denotes the appliedvoltage, G denotes an inter-electrode gap, and S denotes the opposedelectrode area.

In other words, the electrostatic attractive force F is inverselyproportional to the square of the gap G (the second gap G2) between thefirst and second electrodes. Therefore, in the area where the gap Gbetween the first and second electrodes is small, the variation ΔF inthe electrostatic attractive force with respect to the gap variation ΔGis large, and the electrostatic attractive force F drastically varies inresponse to only a minute variation in the gap G, and therefore, gapcontrol for obtaining a predetermined amount of the electrostaticattractive force F is extremely difficult. In contrast thereto, bysetting the inter-electrode gap G to be larger than the gap between thefirst and second reflecting films as in the case of this aspect of theinvention, the variation in the electrostatic attractive force F withrespect to the unit variation in the inter-electrode gap can be reduced.Therefore, it becomes possible to make the level of the electrostaticforce F easy to control.

According to another aspect of the invention, in the optical filteraccording to the above aspect of the invention, it is possible that thefirst opposed surface of the first substrate includes a first surfaceand a second surface disposed in a periphery of the first surface in theplan view, and having a step with the first surface, the first surfaceis provided with the first reflecting film, and the second surface isprovided with the first electrode.

In other words, by providing the step to the first opposed surface ofthe first substrate, the initial gap between the first reflecting filmand the second reflecting film can be formed so as to be smaller thanthe initial gap between the first electrode and the second electrode. Itshould be noted that in this case, since the first and second substratesare the references of a pair of opposed substrates at least one of whichis movable, one having the opposed surface provided with the step can becalled the first substrate, and the other provided with no step can becalled the second substrate. The first substrate can be a fixedsubstrate or a movable substrate.

According to another aspect of the invention, in the optical filteraccording to the above aspect of the invention, it is possible that thefirst opposed surface of the first substrate includes a first surfaceand a second surface disposed in a periphery of the first surface in theplan view, and having a step with the first surface, the first surfaceis provided with the first reflecting film, the second surface isprovided with the first electrode, the second opposed surface of thesecond substrate includes a third surface and a fourth surface disposedin a periphery of the third surface in the plan view, and having a stepwith the third surface, the third surface is provided with the secondreflecting film, and the fourth surface is provided with the secondelectrode.

In other words, by providing the step to both of the first opposedsurface of the first substrate and the second opposed surface of thesecond substrate, the initial gap between the first reflecting film andthe second reflecting film can be formed so as to be smaller than theinitial gap between the first electrode and the second electrode.

According to another aspect of the invention, in the optical filteraccording to the above aspect of the invention, it is possible that thesecond substrate is movably supported with respect to the firstsubstrate, and the second substrate has an area where the secondreflecting section is disposed that has a thickness larger than athickness of an area where the second electrode is disposed.

By forming the area where the second reflecting film is formed to be athick-wall area in such a manner as described above to be hard todeflect, it becomes possible for the second reflecting film to vary thegap while maintaining the parallelism. On this occasion, in the case ofproviding the step to the second substrate, it is possible to form thearea where the second reflecting film is disposed to be the thick-wallarea using the step. It should be noted that since the area where thesecond electrode is formed can be formed to be a thin-wall area, thebendability of the second substrate can be assured.

According to another aspect of the invention, in the optical filteraccording to the above aspect of the invention, it is possible that thefirst electrode is divided into K (K is an integer equal to or greaterthan 2) segment electrodes electrically isolated from each other, andthe second electrode is a common electrode having the same electricalpotential.

In this optical filter, a common voltage (e.g., a ground voltage) isapplied to the second electrode disposed in the periphery of the secondreflecting film in the plan view, and independent voltages arerespectively applied to the K segment electrodes constituting the firstelectrode disposed in the periphery of the first reflecting film in theplan view, thereby varying the dimensions of the gap between the firstand second reflecting films. The applied voltages are direct-currentvoltages. By varying the two parameters, namely the amplitude of thevoltage to be applied to each of the K segment electrodes and the numberof segment electrodes selected for applying the voltages out of the Ksegment electrodes, in such a manner as described above, the dimensionof the gap between the first and second reflecting films is controlled.

It is difficult to obtain both the large gap variable range and the lowsensitivity to the voltage variation due to noise or the like with theparameter of the type of voltage alone as in the case of JP-A-11-142752.By adding the parameter of the number of electrodes as in this aspect ofthe invention, it becomes possible to generate more fine-tunedelectrostatic attractive force to thereby perform the fine gapadjustment in a larger gap variable range by applying the appliedvoltage range the same as in the case of controlling it by voltage aloneto the individual segment electrodes.

Here, it is assumed that the maximum value of the applied voltage isVmax, and the gap can be varied in N levels. In the case in which thefirst electrode is not divided into a plurality of segments, it isnecessary to divide the maximum voltage Vmax into N to thereby assignthe applied voltages. On this occasion, it is assumed that the minimumvalue of the voltage variation between the applied voltages differentfrom each other is ΔV1min. In contrast, in the present embodiment, theapplied voltage to each of the K segment electrodes can be assigned bydividing the maximum voltage Vmax into averagely (N/K). On thisoccasion, it is assumed that the minimum value of the voltage variationbetween the applied voltages different from each other applied to thesame segment with respect to each of the K segment electrodes is ΔVkmin.In this case, it is obvious that ΔV1min<ΔVkmin is true.

In other words, as a result of distributing each of the applied voltagesto the K segment electrodes taking the maximum supply voltage suppliedto the electrical potential difference control section as a full-scale,the minimum value ΔVkmin of the voltage variation between the appliedvoltages to be applied to the same segment electrode can be maderelatively large. As a comparison, compared to the minimum voltagevariation ΔV1min between the applied voltages of N levels in the case offorming the first electrode as a single electrode unlike this aspect ofthe invention, it is obvious that ΔV1min<ΔVkmin is true. As describedabove, if the minimum voltage variation can be assured to be large, thegap variation can be reduced even when the applied voltages to thesegment electrodes vary in a certain extent due to noise depending onthe power supply variation, the environment, and so on. In other words,the sensitivity to noise becomes low, or the voltage sensitivity becomeslower. Thus, gap control with high accuracy becomes possible, andfeedback control on the gap is not necessarily required as inJP-A-11-142752. Further, even if the feedback control is performed onthe gap, since the sensitivity to noise is low, early settling can beachieved.

According to another aspect of the invention, in the optical filteraccording to the above aspect of the invention, it is possible that thefirst electrode is divided into K (K is an integer equal to or greaterthan 2) segment electrodes electrically isolated from each other, thesecond is a common electrode having the same electrical potential, the Ksegment electrodes include at least first and second segment electrodesrespectively including ring-like electrode sections disposed so as tohave a concentric ring shape with respect to a center of the firstreflecting film, and the first segment electrode is disposed innercircumferential side of the second segment electrode.

According to this configuration, the first and second segment electrodeseach become in an axisymmetric arrangement with respect to the verticalcenter line of the first and second reflecting films. Therefore, bymaking the gap variable drive force (the electrostatic attractive force)act symmetrically with respect to the first and second reflecting films,the first and second reflecting films vary the gap therebetween whilekeeping the parallelism.

According to another aspect of the invention, in the optical filteraccording to the above aspect of the invention, it is possible that afirst lead wire is connected to the first segment electrode, the secondsegment electrode is provided with a first slit adapted to make thering-like electrode section of the second segment electrodediscontinuous, and the first lead wire is drawn outside the secondsegment electrode via the first slit.

As described above, in the case of making the first and second segmentelectrodes respectively have the ring-like electrode sections, ataking-out path for the first lead wire of the first segment electrodelocated inside can easily be assured by the first slit provided to thesecond segment electrode located outside.

According to another aspect of the invention, in the optical filteraccording to the above aspect of the invention, it is possible that thesecond electrode provided to the second substrate displaced with respectto the first substrate has third and fourth segment electrodesrespectively including ring-like electrode sections disposed so as tohave a concentric ring shape with respect to a center of the secondreflecting film, the third segment electrode is opposed to the firstsegment electrode, the fourth segment electrode is opposed to the secondsegment electrode, and the third and fourth segment electrodes areelectrically connected to each other.

According to this configuration, since the area of the electrodeprovided to the second substrate, which is movable, is reduced to aminimum, the rigidity of the second substrate is reduced, and thebendability thereof can be assured.

According to another aspect of the invention, in the optical filteraccording to the above aspect of the invention, it is possible that thering-like electrode section of the fourth segment electrode is formedcontinuously at a position opposed to the first slit. Since the firstlead wire is disposed in the first slit, it is possible to generate theelectrostatic attractive force, which acts between the first lead wirehaving the same electrical potential as the first segment electrodelocated inside and the fourth segment electrode located outside, insidethe first slit. As an advantage derived therefrom, if the first andsecond segment electrodes are driven with substantially the samevoltages, for example, even electrostatic attractive force can begenerated in roughly the entire circumference of the fourth segmentelectrode located outside.

According to another aspect of the invention, instead of the feature ofthe optical filter according to the above aspect of the invention, it ispossible that the fourth segment electrode is provided with a secondslit adapted to make the ring-like electrode section of the fourthsegment electrode discontinuous at a position opposed to the first slit.According to this configuration, the electrode opposed to the first leadwire is eliminated. Therefore, it is possible to prevent unwantedelectrostatic attractive force, which acts between the first lead wirehaving the same electric potential as the first segment electrodelocated inside and the fourth segment electrode located outside, frombeing generated in the first slit when driving the first segmentelectrode located inside alone, for example.

According to another aspect of the invention, in the optical filteraccording to the above aspect of the invention, it is possible that atleast one of the first and second substrates is formed as a rectangularsubstrate having first and second diagonal lines. In this case, it ispossible that the first lead wire extends in a first direction along thefirst diagonal line from the first segment electrode, a second lead wireextending in a second direction on the first diagonal line opposite tothe first direction is connected to the second segment electrode, athird lead wire extending in a third direction along the second diagonalline is connected to the third and fourth segment electrodes so as toconnect the third and fourth segment electrodes to each other, a fourthlead wire extending in a fourth direction on the second diagonal lineopposite to the third direction is connected to the third and fourthsegment electrodes so as to connect the third and fourth segmentelectrodes to each other, and first through fourth connection electrodesections to which the first through fourth lead wires are respectivelyconnected are disposed at four corners of the rectangular substrate inthe plan view.

According to this configuration, the first and second lead wiresprovided to the first substrate and the third and fourth lead wiresprovided to the second substrate do not overlap with each other in theplan view, and therefore, no parallel electrodes are constituted.Therefore, no wasteful electrostatic attractive force is generatedbetween the first and second lead wires and the third and fourth leadwires, and further no wasteful capacitance is provided. Further, thewiring lengths of the first through fourth lead wires respectively tothe first through fourth external connection electrode sections becomethe shortest. Therefore, the wiring resistances and the wiringcapacitances of the first through fourth lead wires are reduced, and thecharging/discharging rate of the first through fourth segment electrodescan be raised.

According to another aspect of the invention, in the optical filteraccording to the above aspect of the invention, it is possible that aring width of the second segment electrode is made larger than a ringwidth of the first segment electrode.

Since the electrostatic attractive force is proportional to the area ofthe electrode, the electrostatic attractive force generated by thesecond segment electrode can be increased. This is because it is desiredthat the electrostatic attractive force generated by the second segmentelectrode located outside is greater than the electrostatic attractiveforce generated by the first segment electrode located inside.

According to another aspect of the invention, in the optical filteraccording to the above aspect of the invention, it is possible that thesecond surface of the first substrate includes a 2-1 surface, and a 2-2surface disposed in a periphery of the 2-1 surface in the plan view, andhaving a step with the 2-1 surface, the first segment electrode isdisposed on the 2-1 surface, the second segment electrode is disposed onthe 2-2 surface, and an initial gap between the second segment electrodeand the second electrode is made different from an initial gap betweenthe first segment electrode and the second electrode.

Here, if it is assumed that the 2-1 surface and the 2-2 surface arecoplanar with each other and the initial values of the respective gapsare the same, the inter-electrode gap of either one of the first andsecond segment electrodes, which is driven first, becomes larger thanthe inter-electrode gap of the segment electrode, which is driven later.This is because the inter-electrode gap of the segment electrode drivenlater is reduced in conjunction with the inter-electrode gap of thesegment electrode driven first. Therefore, either one of the first andsecond segment electrodes driven first requires the initialelectrostatic attractive force to be set excessively strong accordinglyto the increment in the initial gap in comparison with the segmentelectrode driven later. By making the initial values of the respectiveinter-electrode gaps different from each other, such a harmful influencecan be suppressed.

According to another aspect of the invention, in the optical filteraccording to the above aspect of the invention, it is possible that aninitial gap between the second segment electrode and the secondelectrode is made smaller than an initial gap between the first segmentelectrode and the second electrode.

As described later, it is advantageous to first drive the second segmentelectrode located outside, and the initial gap between the secondsegment electrode and the second electrode can be reduced in accordancetherewith.

According to another aspect of the invention, in the optical filteraccording to the above aspect of the invention, it is possible that anelectrical potential difference control section adapted to control anelectrical potential difference between each of the K segment electrodesand the second electrode is further provided, and the electricalpotential difference control section applies a plurality of voltagevalues set for each of the K segment electrodes to each of the K segmentelectrodes to thereby switch each of the electrical potentialdifferences from a first electrical potential difference to a secondelectrical potential difference larger than the first electricalpotential difference, then to a third electrical potential differencelarger than the second electrical potential difference, the firstthrough third electrical potential differences being set for each of theK segment electrodes.

As described above, the electrical potential difference control sectionswitches the electrical potential difference of each of the K segmentelectrodes and the second electrode in at least three levels ofelectrical potential difference so that the electrical potentialdifference increases monotonically. Thus, the gap between the first andsecond reflecting films is varied in at least 3×K levels to thereby varythe transmission peak wavelength. In other words, the first electricalpotential difference, the second electrical potential difference, andthe third electrical potential difference set to each of the K segmentelectrodes are determined so as to obtain the gaps between the first andsecond reflecting films with which the respective desired transmissionpeak wavelengths are realized.

Here, if the electrical potential difference is switched, for example,from the second electrical potential difference to the first electricalpotential difference smaller than the second electrical potentialdifference, since the restoring force corresponding to the secondelectrical potential difference is stronger than the electrostaticattractive force corresponding to the first electrical potentialdifference, the time of the damped free vibration of the substrate dueto occurrence of an overshoot and so on becomes longer, and therefore, aprompt wavelength variation operation is not achievable. In contrastthereto, since the electrical potential difference control sectionswitches the electrical potential difference from the first electricalpotential difference to the second electrical potential differencelarger than the first electrical potential difference, and further tothe third electrical potential difference larger than the secondelectrical potential difference, the damped free vibration of thesubstrate can be suppressed, and the prompt wavelength variationoperation can be performed.

According to another aspect of the invention, in the optical filteraccording to the above aspect of the invention, it is possible that anelectrical potential difference control section adapted to respectivelycontrol an inner electrical potential difference between the firstsegment electrode and the second electrode and an outer electricalpotential difference between the second segment electrode and the secondelectrode is further provided, and the electrical potential differencecontrol section applies a plurality of voltage values set for each ofthe first and second segment electrodes to each of the first and secondsegment electrodes to thereby switch each of the inside and outerelectrical potential differences from a first electrical potentialdifference to a second electrical potential difference larger than thefirst electrical potential difference, then to a third electricalpotential difference larger than the second electrical potentialdifference, the first through third electrical potential differencesbeing set for each of the first and second segment electrodes.

By controlling the voltage values applied to the first and secondsegment electrodes adjacent to each other in a radial direction out ofthe K segment electrodes as described above, each of the innerelectrical potential difference and the outer electrical potentialdifference is switched between at least three levels of electricalpotential difference so that the electrical potential differenceincreases monotonically.

According to another aspect of the invention, in the optical filteraccording to the above aspect of the invention, it is possible that theelectrical potential difference control section applies a first segmentvoltage to the first segment electrode in the case of setting the innerelectrical potential difference to the first electrical potentialdifference, a second segment voltage to the first segment electrode inthe case of setting the inner electrical potential difference to thesecond electrical potential difference, a third segment voltage to thefirst segment electrode in the case of setting the inner electricalpotential difference to the third electrical potential difference, afourth segment voltage to the second segment electrode in the case ofsetting the outer electrical potential difference to the firstelectrical potential difference, a fifth segment voltage to the secondsegment electrode in the case of setting the outer electrical potentialdifference to the second electrical potential difference, and a sixthsegment voltage to the second segment electrode in the case of settingthe outer electrical potential difference to the third electricalpotential difference.

As described above, when setting each of the inner electrical potentialdifference and the outer electrical potential difference to the firstthrough third electrical potential differences (i.e., the first throughthird electrical potential differences as the inner electrical potentialdifference and the first through third electrical potential differencesas the outer electrical potential difference are not necessarily equalto each other) set to each of the first and second segment electrodes,the first through third segment voltages are applied to the firstsegment electrode, and the fourth through sixth segment voltages areapplied to the second segment electrode. The applied voltages to beapplied to the first and second segment electrodes are determined basedon the inner and outer electrical potential differences for obtainingthe gaps between the first and second reflecting films with which thedesired transmission peak wavelengths are realized.

According to another aspect of the invention, in the optical filteraccording to the above aspect of the invention, it is possible that withrespect to each of the inner electrical potential difference and theouter electrical potential difference, an absolute value of a differencebetween the second electrical potential difference and the thirdelectrical potential difference is made smaller than an absolute valueof a difference between the first electrical potential difference andthe second electrical potential difference.

The electrostatic attractive force is proportional to the square of theelectrical potential difference. Therefore, when switching theelectrical potential difference in the ascending direction of theelectrical potential difference, namely to the first electricalpotential difference, the second electrical potential difference, andthen the third electrical potential difference, if the absolute value ofthe difference between the first electrical potential difference and thesecond electrical potential difference and the absolute value of thedifference between the second electrical potential difference and thethird electrical potential difference are the same, it results that theelectrostatic attractive force increases drastically, which causes theovershoot. Therefore, it is arranged that the absolute value of thedifference between the second electrical potential difference and thethird electrical potential difference is made smaller than the absolutevalue of the difference between the first electrical potentialdifference and the second electrical potential difference. Thus, it ispossible to suppress the rapid increase in the electrostatic attractiveforce when the gap is narrowed to thereby further suppress theovershoot, and thus, the prompter wavelength variation operation can berealized. It should be noted that the amplitude of the absolute value ofthe difference between the electrical potential differences isdetermined depending on the dimension of the gap between the first andsecond reflecting films corresponding to the desired measurementwavelength, the rigidity of the movable substrate, the substrate areaand the substrate thickness corresponding to each of the areas of thefirst and second reflecting films, and so on.

According to another aspect of the invention, in the optical filteraccording to the above aspect of the invention, it is possible that withrespect to each of the inner electrical potential difference and theouter electrical potential difference, a period during which theelectrical potential difference is set to the second electricalpotential difference is made longer than a period during which theelectrical potential difference is set to the first electrical potentialdifference, and a period during which the electrical potentialdifference is set to the third electrical potential difference is madelonger than a period during which the electrical potential difference isset to the second electrical potential difference.

When the second electrical potential difference larger than the firstelectrical potential difference is set, or the third electricalpotential difference larger than the second electrical potentialdifference is set, the restoring force of the substrate increases, andtherefore, it might take a longer time until the substrate stops. Inother words, the time period before the gap between the first and secondreflecting films settles in place might be longer in some cases. Incontrast, by setting the period set for the second electrical potentialdifference longer than the period set for the first electrical potentialdifference, and setting the period set for the third electricalpotential difference longer than the period set for the secondelectrical potential difference, it is possible to settle the gap at apredetermined value. It should be noted that the length of the voltageapplication period is determined depending on the dimension of the gapbetween the first and second reflecting films corresponding to thedesired measurement wavelength, the rigidity of the movable substrate,the substrate area and the substrate thickness corresponding to each ofthe areas of the first and second reflecting films, and so on.

According to another aspect of the invention, in the optical filteraccording to the above aspect of the invention, it is possible that thegap between the first reflecting film and the second reflecting film isset to a first distance in the case in which the electrical potentialdifference control section sets the outer electrical potentialdifference to the first electrical potential difference, the gap betweenthe first reflecting film and the second reflecting film is set to asecond distance smaller than the first distance in the case in which theelectrical potential difference control section sets the outerelectrical potential difference to the second electrical potentialdifference, the gap between the first reflecting film and the secondreflecting film is set to a third distance smaller than the seconddistance in the case in which the electrical potential differencecontrol section sets the outer electrical potential difference to thethird electrical potential difference, and an absolute value of adifference between the first distance and the second distance is setsubstantially equal to an absolute value of a difference between thesecond distance and the third distance.

By varying the dimension of the gap between the first and secondreflecting films to the first distance, the second distance, and thenthe third distance so as to monotonically decrease by a constant amount,the transmission peak wavelength is also shortened by a constant value.

According to another aspect of the invention, in the optical filteraccording to the above aspect of the invention, it is possible that theelectrical potential difference control section changes the innerelectrical potential difference while keeping the outer electricalpotential difference at the third electrical potential difference, thegap between the first reflecting film and the second reflecting film isset to a fourth distance in the case in which the electrical potentialdifference control section sets the inner electrical potentialdifference to the first electrical potential difference, the gap betweenthe first reflecting film and the second reflecting film is set to afifth distance smaller than the fourth distance in the case in which theelectrical potential difference control section sets the innerelectrical potential difference to the second electrical potentialdifference, the gap between the first reflecting film and the secondreflecting film is set to a sixth distance smaller than the fifthdistance in the case in which the electrical potential differencecontrol section sets the inner electrical potential difference to thethird electrical potential difference, and an absolute value of adifference between the fourth distance and the fifth distance is setsubstantially equal to an absolute value of a difference between thefifth distance and the sixth distance.

By varying the inner electrical potential difference to the firstthrough third electrical potential differences while keeping the outerelectrical potential difference at the third electrical potentialdifference in such a manner as described above, the dimension of the gapbetween the first and second reflecting films can be varied to thefourth distance smaller than the third distance, the fifth distance, andthen the sixth distance so as to be monotonically narrowed by a constantamount, and thus the transmission peak wavelength is also shortened by aconstant value.

It should be noted that the dimension of the gap between the first andsecond reflecting films is more significantly affected by theelectrostatic attractive force based on the inner electrical potentialdifference than the outer electrical potential difference. Therefore, ifthe inner electrical potential difference is firstly varied, and thenthe outer electrical potential difference is varied while keeping theinner electrical potential difference at a constant value, since theelectrostatic attractive force by the inner electrical potentialdifference is dominant, the gap between the first and second reflectingfilms does not vary so largely as the outer electrical potentialdifference varies. Therefore, the outer electrical potential differenceis varied first, and then the inner electrical potential difference isvaried while keeping the outer electrical potential difference at aconstant value.

According to another aspect of the invention, in the optical filteraccording to the above aspect of the invention, it is possible that theelectrical potential difference control section changes the innerelectrical potential difference after the third electrical potentialdifference as the outer electrical potential difference reaches amaximum outer electrical potential difference, while keeping the outerelectrical potential difference at the maximum outer electricalpotential difference.

According to this process, a further gap variation corresponding to onestep from the gap between the first and second reflecting films set bythe outer maximum electrical potential difference becomes possible dueto the application of the inner electrical potential difference.Moreover, since the outer maximum electrical potential difference hasalready been reached, it is not required to further vary the outerelectrical potential difference after the inner electrical potentialdifference is applied. Since it becomes unnecessary to vary the outerelectrical potential difference after varying the inner electricalpotential difference as described above, the harmful influence by thedominant electrostatic attractive force due to the inner electricalpotential difference can be eliminated when varying the outer electricalpotential difference.

According to another aspect of the invention, in the optical filteraccording to the above aspect of the invention, it is possible that thegap between the first reflecting film and the second reflecting film isset to a minimum distance when the electrical potential differencecontrol section sets the third electrical potential difference as theinner electrical potential difference to a maximum inner electricalpotential difference, and the maximum outer electrical potentialdifference and the maximum inner electrical potential difference aremade substantially equal to each other within a range of a maximumsupply voltage supplied to the electrical potential difference controlsection.

According to the configuration described above, since each the appliedvoltages to the respective first and second segment electrodes can bedistributed taking the maximum supply voltage to be supplied to theelectrical potential difference control section as the full-scale, theminimum voltage variation described above can be made larger than in therelated art. Therefore, sensitivity to noise can be reduced.

According to another aspect of the invention, in the optical filteraccording to the above aspect of the invention, it is possible that theelectrical potential difference control section applies voltagessequentially to the respective K segment electrodes to thereby vary thegap between the first reflecting film and the second reflecting film inN levels in total, and in comparison between a minimum value ΔVkmin of avoltage difference between applied voltages to be applied to the samesegment electrode out of the K segment electrodes and a minimum voltagevariation ΔV1min between N levels of applied voltages in the case offorming the first electrode as a single electrode, ΔV1min<ΔVkmin istrue. Thus, the sensitivity to noise can be reduced as described above.

According to another aspect of the invention, there is defined ananalytical instrument including any one of the optical filters describedabove. As an analytical instrument of this kind, the light beamreflected, absorbed, transmitted, or emitted by the analysis object ismade to input a variable wavelength optical filter, the light beams withrespective wavelengths transmitted through the optical filter arereceived by the light receiving element, and the signal from the lightreceiving element is operated by an arithmetic circuit, therebymeasuring the intensity of the light beams with the respectivewavelength, for example, thus the color, mixture component in the gas,and so on can be analyzed.

According to still another aspect of the invention, there is defined anoptical apparatus including any one of the optical filters describedabove. As an optical apparatus of this kind, there can be cited atransmitter of an optical multiplexing communication system such as anoptical code division multiplexing (OCDM) transmitter or a wavelengthdivision multiplexing (WDM) transmitter. In the WDM, the channels arediscriminated by the wavelength of the optical pulse constituting theoptical pulse signals. Although in the OCDM the channels arediscriminated by pattern matching of encoded optical pulse signals, theoptical pulses constituting the optical pulse signals include lightcomponents with respective wavelengths different from each other.Therefore, in the transmitter of the optical multiplexing communicationsystem, light beams with a plurality of wavelengths are used, and byusing the optical filter according to any one of the above aspects ofthe invention, a plurality of light beams with respective wavelengthscan be obtained from a light beam emitted from a single light source.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described with reference to the accompanyingdrawings, wherein like numbers reference like elements.

FIG. 1 is a cross-sectional view showing a non-voltage application stateof an optical filter according to an embodiment of the invention.

FIG. 2 is a cross-sectional view showing a voltage application state ofthe optical filter shown in FIG. 1.

FIG. 3 is a characteristics diagram showing a relationship betweenelectrostatic attractive force and an inter-electrode gap.

FIG. 4A is a plan view of a second electrode, and FIG. 4B is a plan viewof a first electrode of the embodiment.

FIGS. 5A and 5B are plan views of the first and second electrodes in anoverlapping state viewed from the side of a second substrate.

FIG. 6 is a plan view showing a wiring layout of first through fourthlead wires viewed from the side of the second substrate through thesecond substrate.

FIG. 7 is a block diagram of an applied voltage control system of theoptical filter.

FIG. 8 is a characteristics table showing an example of voltage tabledata.

FIG. 9 is a timing chart of voltage application realized with thevoltage table data.

FIG. 10 is a graph showing a relationship between a gap between firstand second reflecting films of the optical filter and a transmissionpeak wavelength.

FIG. 11 is a graph showing a relationship between an electricalpotential difference between the first and second electrodes and theelectrostatic attractive force.

FIG. 12 is a characteristics table showing data of the embodimentregarding the electrical potential difference, the gap, and the variablewavelength shown in FIG. 8.

FIG. 13 is a graph showing a relationship between the applied voltageand the gap shown in FIG. 12.

FIG. 14 is a graph showing a relationship between the applied voltageand the transmission peak wavelength shown in FIG. 12.

FIGS. 15A and 15B are plan views showing first and second electrodes ofa comparative example.

FIG. 16 is a characteristics table showing data of the comparativeexample related to the electrical potential difference, the gap, and thevariable wavelength.

FIG. 17 is a graph showing a relationship between the applied voltageand the gap shown in FIG. 16.

FIG. 18 is a graph showing a relationship between the applied voltageand the transmission peak wavelength shown in FIG. 16.

FIG. 19 is a cross-sectional view showing non-voltage application stateof an optical filter according to another embodiment of the invention.

FIG. 20 is a block diagram of an analytical instrument according tostill another embodiment of the invention.

FIG. 21 is a flowchart showing a spectral measurement operation in theinstrument shown in FIG. 20.

FIG. 22 is a block diagram of an optical apparatus according to stillanother embodiment of the invention.

FIG. 23 is a cross-sectional view of an optical filter according tostill another embodiment of the invention having the movable substrateprovided with a step.

FIG. 24 is a cross-sectional view of an optical filter according tostill another embodiment of the invention having both of the first andsecond substrates each provided with a step.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

Hereinafter, some preferred embodiments of the invention will bedescribed in detail. It should be noted that the embodiments explainedbelow do not unreasonably limit the content of the invention as setforth in the appended claims, and all of the constituents set forth inthe present embodiments are not necessarily essential to the invention.

1. Optical Filter 1.1. Filter Section of Optical Filter 1.1.1. GeneralDescription of Filter Section

FIG. 1 is a cross-sectional view of an optical filter 10 according tothe present embodiment in a non-voltage application state, and FIG. 2 isa cross-sectional view thereof in a voltage application state. Theoptical filter 10 shown in FIGS. 1 and 2 includes a first substrate 20and a second substrate 30 opposed to the first substrate 20. Although inthe present embodiment it is assumed that the first substrate 20 is afixed substrate, and the second substrate 30 is a movable substrate ordiaphragm, it is sufficient that either one or both of the substratesare movable.

In the present embodiment, there is provided a support section 23formed, for example, integrally with the first substrate 20, and formovably supporting the second substrate 30. The support section 20 canalso be provided to the second substrate 30, or can be formed separatelyfrom the first and second substrates 20, 30.

The first and second substrates 20, 30 can each be made of various typesof glass such as soda glass, crystalline glass, quartz glass, leadglass, potassium glass, borosilicate glass, or alkali-free glass, aquartz crystal, or the like. Among these materials, as the constituentmaterial of the substrates 20, 30 the glass containing alkali metal suchas sodium (Na) or potassium (K) is preferable, and by forming thesubstrates 20, 30 using such glass materials, the adhesiveness withreflecting films 40, 50 and electrodes 60, 70 described later, and thebonding strength between the substrates can be improved. Further, thesetwo substrates 20, 30 are bonded by, for example, surface activatedbonding with a plasma-polymerized film to thereby be integrated witheach other. Each of the first and second substrates 20, 30 has a squareshape, for example 10 mm on a side, and the greatest diameter of theportion functioning as a diaphragm is, for example, 5 mm.

The first substrate 20 is formed by etching a glass substrate that has athickness of, for example, 500 μm. The first substrate 20 is providedwith a first reflecting film 40 having, for example, a circular shapeformed on a first surface 20A1 located at a central portion of the firstopposed surface 20A opposed to the second substrate 30. Similarly, thesecond substrate 30 is formed by etching a glass substrate that has athickness of, for example, 200 μm. The second substrate 30 is providedwith a second reflecting film 50, which has, for example, a circularshape and is opposed to the first reflecting film 40, formed at acentral position of a second opposed surface 30A opposed to the firstsubstrate 20.

It should be noted that the first and second reflecting films 40, 50 areeach formed to have, for example, a circular shape with a diameter ofabout 3 mm. The first and second reflecting films 40, 50 are each areflecting film formed of an AgC single layer, and can be providedrespectively to the first and second substrates 20, 30 by a method suchas sputtering. The AgC single layer reflecting film has a thicknessdimension of, for example, 0.03 μm. Although in the present embodimentthere is described an example of using the reflecting film of the AgCsingle layer capable of performing a dispersion operation in the entirevisible light range as the first and second reflecting films 40, 50, thereflecting films are not limited thereto. It is also possible to use adielectric multilayer film obtained by stacking laminated films of, forexample, TiO₂ and SiO₂, which can perform the dispersion operation in anarrower wavelength band, but has a higher transmittance of thedispersed light beams, a narrower half-value width of the transmittance,and more preferable resolution compared to the AgC single layerreflecting film.

Further, it is possible to form antireflection films (AR) not shown onthe respective surfaces of the first and second substrates 20, 30 on theopposite side to the first and second opposed surfaces 20A, 30A thereofat positions corresponding to the first and second reflecting films 40,50. The antireflection films are each formed by alternately stacking lowrefractive index films and high refractive index films, and decrease thereflectance to the visible light on the interfaces of the first andsecond substrates 20, 30 while increasing the transmittance thereof.

The first and second reflecting films 40, 50 are disposed so as to beopposed to each other via a first gap G1 in the non-voltage applicationstate shown in FIG. 1. It should be noted that although in the presentembodiment a fixed mirror is used as the first reflecting film 40 and amovable mirror is used as the second reflecting film 50, it is possibleto make either one or both of the first and second reflecting filmsmovable in accordance with the configuration of the first and secondsubstrates 20, 30 described above.

The second surface 20A2, which is located on the periphery of the firstreflecting film 40 and on the periphery of the first surface 20A1 of thefirst substrate 20 in the plan view, is provided with the firstelectrode 60. Similarly, the second opposed surface 30A of the secondsubstrate 30 is provided with the second electrode so as to be opposedto the first electrode 60. The first electrode 60 and the secondelectrode 70 are disposed so as to be opposed to each other via a secondgap G2. It should be noted that the surfaces of the first and secondelectrodes 60, 70 can be covered by an insulating film.

In the present embodiment, the first opposed surface 20A of the firstsubstrate 20 opposed to the second substrate 30 includes the firstsurface 20A1 provided with the first reflecting film 40 and the secondsurface 20A2 disposed in the periphery of the first surface 20A1 in theplan view, and provided with the first electrode 60. The first surface20A1 and the second surface 20A2 are not coplanar with each other, thereis a step 22 between the first surface 20A1 and the second surface 20A2,and the first surface 20A1 is placed nearer to the second substrate 30than the second surface 20A2. Thus, the relationship of (first gapG1)<(second gap G2) becomes true with initial values in the non-voltageapplication state.

Here, according to Formula 1 described above, the electrostaticattractive force F is inversely proportional to the square of the gap G(the second gap G2) between the first and second electrodes 60, 70. FIG.3 shows a relationship between the variation ΔF in the electrostaticattractive force F and the variation ΔG in the gap G between the firstand second electrodes 60, 70. FIG. 3 shows a gap variation ΔG1 in anarea where the inter-electrode gap G is small and a gap variation ΔG2(=ΔG1) in an area where the inter-electrode gap G is large. In the areawhere the inter-electrode gap G is small, only by varying the gap assmall as the gap variation ΔG1, the electrostatic attractive force Fvaries as large as ΔF1. In contrast thereto, in the area where theinter-electrode gap G is large, even in the case in which the gap variesas much as the gap variation ΔG2 equal to the gap variation ΔG1, thevariation in the electrostatic attractive force F takes a relativelysmall value of ΔF2.

As described above, since in the area where the inter-electrode gap G isrelatively narrow, the electrostatic attractive force F variesdrastically in response to only minute variation in the gap G, andtherefore, gap control for obtaining a predetermined electrostaticattractive force F is extremely difficult. In contrast thereto, in thearea where the inter-electrode gap G is relatively large, the variationin the electrostatic attractive force F per unit gap variation is small.Therefore, it can be understood that it is easier to control the levelof the electrostatic attractive force by the inter-electrode gap G inthe case of using the flat area in the F-G characteristics curve shownin FIG. 3 where the variation in the electrostatic attractive force F issmall.

Incidentally, the transmission wavelength band of the variablewavelength object is, for example, 380 through 700 nm, and the first gapG1 between the first and second reflecting films 40, 50 is as small as140 through 300 nm. Therefore, as shown in FIG. 1, the configuration isset to satisfy (first gap G1)<(second gap G2), thereby making thecontrol of the level of the electrostatic attractive force F easier.

The first electrode 60 is divided into at least K (K is an integer equalto or greater than 2) segment electrodes electrically isolated from eachother, and in the present embodiment, the first electrode 60 can havefirst and second segment electrodes 62, 64 as an example with K=2.Specifically, the K segment electrodes 62, 64 can be set separately tovoltages different from each other, while the second electrode 70 is acommon electrode having the same electrical potential. It should benoted that if K≧3 is satisfied, the relationship between the first andsecond segment electrodes 62, 64 described below can be applied to anytwo segment electrodes adjacent to each other. It should be noted thatthe invention is not necessarily limited to those having the firstelectrode 60 divided into K segment electrodes. An embodiment in whichthe first electrode 60 is not divided will be described later withreference to FIGS. 15A through 18.

In the optical filter 10 having such a structure, the first and secondsubstrates 20, 30 have respective areas where the reflecting films (thefirst and second reflecting films 40, 50) are respectively formed andrespective areas where the electrodes (the first and second electrodes60, 70) are respectively formed as areas different from each other inthe plan view, and there is no chance of stacking the reflecting filmand the electrode with each other as in the case of JP-A-11-142752.Therefore, even if one (the second substrate 30 in the presentembodiment) of the first and second substrates 20, 30 is formed as amovable substrate, the reflecting film and the electrode are not stackedwith each other, and therefore, the movable substrate can be assured tobe easy to deflect. Moreover, unlike JP-A-11-142752, since thereflecting films are not formed on the first and second electrodes 60,70, the restriction of using light transmissive electrodes as the firstand second electrodes 60, 70 does not arise. It should be noted thatsince even the light transmissive electrode affects the transmissioncharacteristics, by eliminating the formation of the reflecting films onthe first and second electrodes 60, 70, the optical filter 10 as atransmissive or reflective variable wavelength interference filter canbe provided with desired transmission or reflection characteristics.

Further, in the optical filter 10 by applying the common voltage (e.g.,the ground voltage) to the second electrode 70 disposed in the peripheryof the second reflecting film 50 in the plan view, and applying voltagesindependent of each other to the respective K segment electrodes 62, 64constituting the first electrode 60 disposed in the periphery of thefirst reflecting film in the plan view to thereby make the electrostaticattractive forces F1, F2 indicated by the respective arrows act betweenthe opposed electrodes as shown in FIG. 2, the first gap G1 between thefirst and second reflecting films 40, 50 can be varied so as to have adimension smaller than the initial gap.

In other words, as shown in FIG. 2 showing the optical filter 10 in thevoltage application state, a first gap variable drive section (anelectrostatic actuator) 80 composed of the first segment electrode 62and the second electrode 70 opposed thereto, and a second gap variabledrive section (an electrostatic actuator) 90 composed of the secondsegment electrode 64 and the second electrode 70 opposed thereto aredriven independently of each other.

As described above, by providing the plurality (K) of gap variable drivesections 80, 90 independent of each other disposed only in the peripheryof the first and second reflecting films 40, 50 in the plan view, andvarying two parameters, namely the values of the voltages appliedrespectively to the K segment electrodes 62, 64 and the number ofsegment electrodes selected for applying the voltage among the K segmentelectrodes 62, 64, the dimension of the gap between the first and secondreflecting films 40, 50 is controlled.

It is difficult to obtain both of the large gap variable range and thelow sensitivity to the voltage variation due to noise or the like withthe parameter of the type of voltage alone as in the case ofJP-A-11-142752. By adding the parameter of the number of electrodes asin the present embodiment, it becomes possible to generate morefine-tuned electrostatic attractive force to thereby perform fine gapadjustment in a larger gap variable range by applying the appliedvoltage range the same as in the case of controlling it by voltage aloneto the individual segment electrodes.

Here, it is assumed that the maximum value of the applied voltage isVmax, and the gap can be varied in N levels. In the case in which thefirst electrode 60 is not divided into a plurality of segments, it isnecessary to divide the maximum voltage Vmax into N to thereby assignthe applied voltages. On this occasion, it is assumed that the minimumvalue of the voltage variation between the applied voltages differentfrom each other is ΔV1min. In contrast, in the present embodiment, theapplied voltage to each of the K segment electrodes can be assigned bydividing the maximum voltage Vmax into averagely (N/K). On thisoccasion, it is assumed that the minimum value of the voltage variationbetween the applied voltages different from each other applied to thesame segment with respect to each of the K segment electrodes is ΔVkmin.In this case, it is obvious that ΔV1min<ΔVkmin becomes true.

As described above, if the minimum voltage variation ΔVkmin can beassured to be large, the gap variation can be reduced even when theapplied voltages to the K segment electrodes 62, 64 vary in a certainextent due to noise depending on the power supply variation, theenvironment, and so on. In other words, the sensitivity to noise becomeslow, or the voltage sensitivity becomes lower. Thus, gap control withhigh accuracy becomes possible, and feedback control on the gap is notnecessarily required as in JP-A-11-142752. Further, even if the feedbackcontrol is performed on the gap, since the sensitivity to noise is low,early settling can be achieved. In the present embodiment, by settingthe areas of the first and second reflecting films 40, 50 on the centerside to undriven areas, and the peripheries thereof to driven areas, theparallelism between the first and second reflecting films 40, 50 ismaintained. The parallelism between the first and second reflectingfilms 40, 50 is an important technology component for a Fabry-Perotinterferometer for attenuating the light beam with an unwantedwavelength using interference by multiply reflecting the light beambetween the first and second reflecting films 40, 50.

In the present embodiment, in order for assuring bendability of thesecond substrate 30 as the movable substrate, an area where the secondelectrode 70 is formed is formed as a thin-wall section 34 with athickness dimension of, for example, 50 μm as shown in FIG. 1. Thethin-wall section 34 has a wall thickness smaller than that of athick-wall section 32 of the area where the second reflecting film 50 isdisposed and that of a thick-wall section 36 of the area having contactwith the support section 23. In other words, in the second substrate 30the second opposed surface 30A provided with the second reflecting film50 and the second electrode 70 is a flat plane, the thick-wall section32 is formed in a first area where the second reflecting film 50 isdisposed, and the thin-wall section 34 is formed in a second area wherethe second electrode 70 is formed. As described above, by making thethick-wall section 32 difficult to bend while assuring the bendabilitywith the thin-wall section 34, it becomes possible to vary the gap whilekeeping the flatness of the second reflecting film 50. An example ofproviding a step section or locally providing a thick-wall section tothe second substrate 30 will be described later.

It should be noted that although in the present embodiment the (K) gapvariable drive sections independent of each other are each constitutedwith an electrostatic actuator composed of a pair of electrodes, it isalso possible to replace at least one of those electrostatic actuatorswith another actuator such as a piezoelectric element. It should benoted that the electrostatic actuator for providing attractive force ina non-contact manner causes little interference between gap variabledrive sections, and is therefore suitable for controlling the gap withhigh accuracy. In contrast thereto, in the case, for example, in whichtwo piezoelectric elements are disposed between the first and secondsubstrates 20, 30, there is caused a phenomenon, for example, that thepiezoelectric element, which is not driven, acts as what hinders the gapvariation caused by the other piezoelectric element, which is driven,and a harmful effect is caused in the method of driving two or more gapvariable drive sections independently of each other. From that point ofview, the plurality of gap variable drive sections is preferablycomposed of the electrostatic actuators.

1.1.2. First Electrode

As shown in FIG. 4A, the K segment electrodes 62, 64 constituting thefirst electrode 60 can be arranged to have a concentric ring shape withrespect to the center of the first reflecting film 40. Specifically, thefirst segment electrode 62 has a first ring-like electrode section 62A,the second segment electrode 64 has a second ring-like electrode section64A outside the first ring-like electrode section 62A, and each of thering-like electrode sections 62A, 64A has a concentric ring shape withrespect to the first reflecting film 40. It should be noted that a “ringshape” is a term not limited to the shape of an endless ring, butincludes a discontinuous ring shape, and is not limited to the shape ofa circular ring, but includes the shapes of rectangular ring, polygonalring, and so on.

According to this configuration, as shown in FIG. 2, the first andsecond segment electrodes 62, 64 become in an axisymmetric arrangementwith respect to the center line L of the first reflecting film 40.According to this configuration, since the electrostatic attractiveforces F1, F2 acting between the first and second electrodes 60, 70 inresponse to application of the voltages acts thereon axisymmetricallywith respect to the center line L of the first reflecting film 40,parallelism between the first and second reflecting films 40, 50 isenhanced.

It should be noted that as shown in FIG. 4A, the ring width W2 of thesecond segment electrode 64 can be set larger than the ring width W1 ofthe first segment electrode 62 (W2>W1). This is because theelectrostatic attractive force is proportional to the area of theelectrode, and the electrostatic attractive force F2 generated by thesecond segment electrode 64 is stronger than the electrostaticattractive force F1 generated by the first segment electrode 62. In moredetail, the second segment electrode 64 located outside is disposednearer to the support section 23 of the substrates functions as a hingesection compared to the first segment electrode 62. Therefore, it isdesired for the second segment electrode 64 to generate theelectrostatic attractive force F2 strong enough to overcome theresistive force at the support section (the hinge section) 23. Thesecond segment electrode 64 located outside has a larger diameter thanthat of the first segment electrode 62 located inside, and therefore,even if the widths are the same (W1=W2), the area of the second segmentelectrode 64 is larger. Therefore, although it is also possible to makethe widths equal to each other (W1=W2), the ring width W2 is made largerto thereby make it possible to further increase the area to increase theelectrostatic attractive force F2 generated by the second segmentelectrode 64. In particular, in the case in which the second segmentelectrode 64 located outside is driven prior to the first segmentelectrode 62 as described later, since the initial gap G2 between thesecond segment electrode 64 and the second electrode 70 is large, it isadvantageous in view of the fact that it is possible to make the area ofthe second segment electrode 64 larger to thereby increase theelectrostatic attractive force F2 generated there. On that occasion,since the gap is made smaller when starting to drive the first segmentelectrode 62 located inside as long as the drive state of the firstsegment electrode 64 is maintained, there is no harmful effect ondriving if the ring width W1 of the first segment electrode 62 is small.

Here, a first lead wire 62B is connected to the first segment electrode62, and a second lead wire 64B is connected to the second segmentelectrode 64. The first and second lead wires 62B, 64B are formed so asto extend in radial directions from the center of the first reflectingfilm 40, for example. There is provided a first slit 64C for making thesecond ring-like electrode section 64A of the second segment electrode64 discontinuous. The first lead wire 62B extending from the firstsegment electrode 62 located inside is drawn to the outside of thesecond segment electrode 64 via the first slit 64C provided to thesecond segment electrode 64 located outside.

As described above, in the case of making the first and second segmentelectrodes 62, 64 respectively have the ring-like electrode sections62A, 64A, a taking-out path for the first lead wire 62B of the firstsegment electrode 62 located inside can easily be assured by the firstslit 64C provided to the second segment electrode 64 located outside.

1.1.3. Second Electrode

The second electrode 70 disposed on the second substrate 30 can beformed as a mat electrode in an area of the second substrate 30including an area opposed to the first electrode 60 (the first andsecond segment electrodes 62, 64) provided to the first substrate 20.This is because the second electrode 70 is the common electrode set tothe same voltage.

Alternatively, the second electrode 70 disposed on the second electrode30 displaced with respect to the first substrate 20 as in the presentembodiment can be divided into K segment electrodes similarly to thefirst electrode 60. The K segment electrodes can also be arranged tohave a concentric ring shape with respect to the center of the secondreflecting film 50. According to this configuration, since the area ofthe electrode provided to the second substrate 30, which is movable, isreduced to a minimum, the rigidity of the second substrate 30 isreduced, and the bendability can be assured.

As shown in FIGS. 1, 2, and 4B, the K segment electrodes constitutingthe second electrode 70 can include the third segment electrode 72 andthe fourth segment electrode 74. The third segment electrode 72 has athird ring-like electrode section 72A, the fourth segment electrode 74has a fourth ring-like electrode section 74A outside the third ring-likeelectrode section 72A, and each of the ring-like electrode sections 72A,74A has a concentric ring shape with respect to the second reflectingfilm. The meaning of the “concentric ring shape” is the same as used forthe first electrode 60. The third segment electrode 72 corresponds tothe first segment electrode 62, and the fourth segment electrode 74corresponds to the second segment electrode 64. Therefore, in thepresent embodiment, the ring width (equal to the ring width W2 of thesecond segment electrode 64) of the fourth segment electrode 74 islarger than the ring width (equal to the ring width W1 of the firstsegment electrode 62) of the third segment electrode 72.

Further, the third and fourth segment electrodes 72, 74 are electricallyconnected to each other to be set to the same electrical potential.Therefore, third and fourth lead electrodes 76A, 76B are formed so as toextend from the center of the second reflecting film 50 in radialdirections, for example. Each of the third and fourth lead electrodes76A, 76B is electrically connected to both the third segment electrode72 located inside and the fourth segment electrode 74 located outside.It should be noted that although the third and fourth segment electrodes72, 74 as a common electrode can be connected with a single leadelectrode, by providing two or more lead electrodes, the wiringresistance can be reduced to thereby improve the charging/dischargingrate of the common electrode.

1.1.4. Overlapping Area Between First and Second Electrodes

FIG. 5A shows an overlapping state in a plan view of the first andsecond electrodes 60, 70 of the present embodiment viewed from the sideof the second substrate 30. In FIG. 5A, since the first electrode 60located on the lower side has the first and second segment electrodes62, 64 opposed to the third and fourth segment electrodes 72, 74 of thesecond electrode 70, the first electrode 60 does not appear in the planview thereof viewed from the side of the second substrate 30. Only thefirst and second lead wires 62B, 64B of the first electrode located onthe lower side appear in the plan view viewed from the side of thesecond substrate 30 as indicated by hatching. Since the third ring-likeelectrode section 74A of the second electrode 70 is continuous in thecircumferential direction, the first lead wire 62B is opposed to theopposed area 74A1 of the third ring-like electrode section 74A in anintermediate area 62B1 thereof.

As shown in FIG. 4A, in the present embodiment since the second segmentelectrode 64 located outside out of the first electrode 20 has a firstslit 64C, the electrostatic attractive force F2 (see FIG. 2) based onthe voltage applied to the second segment electrode 64 does not act inthe area of the slit 64C.

On the other hand, since the first lead wire 62B is disposed in thefirst slit 64C as shown in FIG. 4A, the electrostatic attractive forceF1 (see FIG. 2) acting between the first lead wire 62B having the sameelectrical potential as the first segment electrode 62 located insideand the fourth segment electrode 74 located outside can be generated inthe first slit 64C. As an advantage of this configuration, in the caseof, for example, driving the first and second segment electrodes 62, 64with substantially the same voltages, uniform electrostatic attractiveforce can be generated in substantially the entire circumference(including the opposed area 74A1 to the first slit 64C) of the fourthsegment electrode 74 located outside.

FIG. 5B shows an overlapping state in a plan view of the first andsecond electrodes 60, 70′ as a modified example viewed from the side ofthe second substrate 30. The second electrode 70′ shown in FIG. 5B isdifferent from the second electrode 70 in the point that the fourthsegment electrode 74 is further provided with a second slit 78 formaking the fourth ring-like electrode section 74′ discontinuous at theposition opposed to the first slit 64C of the first electrode 60. Inother points, the second electrode 70′ shown in FIG. 5B is the same asthe second electrode 70 shown in FIG. 5A.

According to this configuration, the electrode opposed to the first leadwire 62B is eliminated. Therefore, it is possible to prevent unwantedelectrostatic attractive force, which acts between the first lead wire62B having the same electrical potential as the first segment electrode62 located inside and the fourth segment electrode 74′ located outside,from being generated in the first slit 64C when driving the firstsegment electrode 62 located inside, for example.

1.1.5. Lead Wires

FIG. 6 is a plan view viewed from the side of the second substrate 30through the second substrate 30, and shows a wiring layout of the firstthrough fourth lead wires 62B, 64B, 76A, and 76B. In FIG. 6, at leastone of the first and second substrates 20, 30 is formed as a rectangularsubstrate having first and second diagonal lines. In the presentembodiment, each of the first and second substrates 20, 30 has a squareshape, for example 10 mm on a side. Assuming that the direction in whichthe first lead wire 62B extends from the first ring-like electrodesection 62A of the first segment electrode 62 along the first diagonalline is a first direction D1, the second lead wire 64B extends on thefirst diagonal line in a second direction D2 which is the reversedirection to the first direction D1. The third lead wire 76A extends ina third direction D3 along the second diagonal line. The fourth leadwire 76B extends on the second diagonal line in a fourth direction D4which is the reverse direction to the third direction D3. Further, thereare disposed first through fourth external connection electrode sections101 through 104 located at four corners of the rectangular substrates20, 30 in the plan view to which the first through fourth lead wires62B, 64B, 76A, and 76B are connected respectively.

According to this configuration, firstly, the first and second leadwires 62B, 64B provided to the first substrate 20 and the third andfourth lead wires 76A, 76B provided to the second substrate 30 do notoverlap with each other in the plan view, and therefore, no parallelelectrodes are constituted. Therefore, no wasteful electrostaticattractive force is generated between the first and second lead wires62B, 64B and the third and fourth lead wires 76A, 76B, and further nowasteful capacitance is provided. Further, the wiring lengths of thefirst through fourth lead wires 62B, 64B, 76A, and 76B respectively tothe first through fourth external connection electrode sections 101through 104 become the shortest. Therefore, the wiring resistances andthe wiring capacitances of the first through fourth lead wires 62B, 64B,76A, and 76B are reduced, and the charging/discharging rate of the firstthrough fourth segment electrodes 62, 64, 72, and 74 can be raised.

It should be noted that it is also possible to provide the first throughfourth external connection electrode sections 101 through 104 to eitherone of the first and second substrates 20, 30, or to provide some of thefirst through fourth external connection electrodes 101 through 104 andthe rest thereof to the respective substrates 20, 30. In the case ofdisposing the first through fourth external connection electrodes 101through 104 to either one of the first and second substrates 20, 30, thelead wire provided to the other of the first and second substrates 20,30 can be connected to the external connection electrode sectionprovided to the one of the substrates via a conductive paste or thelike. It should be noted that the first through fourth externalconnection electrode sections 101 through 104 are connected to theoutside via connection sections such as lead wires or wire bonding.

Further, the first through fourth lead wires 62B, 64B, 76A, and 76B canintersect, for example, a plasma polymeric film for bonding the firstand second substrates 20, 30 to each other. Alternatively, it is alsopossible to draw the first through fourth lead wires 62B, 64B, 76A, and76B to the outside beyond the bonding surface via groove sectionsprovided to one of the bonding surfaces of the first and secondsubstrates 20, 30.

1.2. Voltage Control System of Optical Filter 1.2.1. General Descriptionof Applied Voltage Control System

FIG. 7 is a block diagram of an applied voltage control system of theoptical filter 10. As shown in FIG. 7, the optical filter 10 has anelectrical potential difference control section 110 for controlling theelectrical potential difference between the first electrode 60 and thesecond electrode 70. In the present embodiment, since the secondelectrode 70 (the third and fourth segment electrodes 72, 74) as thecommon electrode is fixed to a constant common voltage, for example, theground voltage (0V), the electrical potential control section 110 variesthe applied voltages to the first and second segment electrodes 62, 64as the K segment electrodes constituting the first electrode 60 tothereby respectively control an inner electrical potential differenceΔVseg1 and an outer electrical potential difference ΔVseg2 between therespective first and second segment electrodes 62, 64 and the secondelectrode 70. It should be noted that the second electrode 70 can beprovided with the common voltage other than the ground voltage, and onthat occasion, it is possible for the electrical potential differencecontrol section 110 to control application/non-application of the commonvoltage to the second electrode 70.

In FIG. 7 the electrical potential control section 110 includes a firstsegment electrode drive section connected to the first segment electrode62 such as a first digital-analog converter (DAC1) 112, a second segmentelectrode drive section connected to the second segment electrode 64such as a second digital-analog converter (DAC2) 114, and a digitalcontrol section 116 for performing control, such as digital control, onthe digital-analog converters. The first and second digital-analogconverters 112, 114 are supplied with voltages from a power supply 120.The first and second digital-analog converters 112, 114 are suppliedwith the voltages from the power supply 120, and at the same time outputanalog voltages corresponding to the digital values from the digitalcontrol section 116. As the power supply 120, what is implemented in ananalytical instrument or an optical apparatus to which the opticalfilter 10 is mounted can be used, and further, a power supply dedicatedto the optical filter 10 can also be used.

1.2.2. Method of Driving Optical Filter

FIG. 8 is a characteristics table showing an example of voltage tabledata, which is original data of the control in the digital controlsection 116 shown in FIG. 7. The voltage table data can be provided tothe digital control section 116 itself, or implemented in the analyticalinstrument or the optical apparatus to which the optical filter 10 ismounted.

FIG. 8 shows an example with N=9 as the voltage table data for varyingthe gap between the first and second reflecting films 40, 50 in N levelsin total by sequentially applying the voltages to each of the K segmentelectrodes 62, 64. It should be noted that in FIG. 8, the case in whichthe respective electrical potential differences between the first andsecond segment electrodes 62, 64 and the second electrode 70 are 0V isnot included in the N levels of gap variable range. FIG. 8 shows onlythe case in which the voltage value other than the voltage value (0V) ofthe common voltage applied to the second electrode 70 is applied to atleast one of the first and second segment electrodes 62, 64. It shouldbe noted that it is also possible to define the case, in which both ofthe electrical potential difference between the first segment electrode62 and the second electrode 70 and the electrical potential differencebetween the second segment electrode 64 and the second electrode 70 are0V, as the maximum transmission peak wavelength.

The electrical potential difference control section 110 sets the voltagevalues to the respective K segment electrodes (the first and secondsegment electrodes 62, 64) in accordance with the voltage table datashown in FIG. 8, and then applies the voltage values to the respective Ksegment electrodes (the first and second segment electrodes 62, 64).FIG. 9 is a timing chart of the voltage application realized byperforming the drive in the order of the data number of the voltagetable data shown in FIG. 8.

As shown in FIGS. 8 and 9, L (=4) levels of voltages (VI1 through VI4;VI1<VI2<VI3<VI4) are applied to the first segment electrode 62, and M(=5) levels of voltages (VO1 through VO5; VO1<VO2<VO3<VO4<VO5) areapplied to the second segment electrode 64, thereby varying the firstgap G1 between the first and second reflecting films 40, 50 in N(N=L+M=9) levels from g0 to g8.

According to the voltage control described above, the optical filter 10can realize the wavelength transmission characteristics shown in FIG.10. FIG. 10 shows the wavelength transmission characteristics in thecase of varying the dimension of the first gap G1 between the first andsecond reflecting films 40, 50 in a range of, for example, g0 throughg3. In the optical filter 10, when the dimension of the first gap G1between the first and second reflecting films 40, 50 is varied in therange of, for example, g0 through g3 (g0>g1>g2>g3), the transmissionpeak wavelength is determined in accordance with the dimension of thefirst gap G1. Specifically, the wavelength λ of the light beamtransmitted through the optical filter 10 satisfies the condition thatthe value obtained by multiplying the half wavelength (λ/2) by aninteger (n) is equal to the dimension of the first gap G1 (n×λ=2G1). Thelight beam having the wavelength λ, which fails to satisfy the conditionthat the value obtained by multiplying the half wavelength (λ/2) by aninteger (n) is equal to the dimension of the first gap G1, interferesitself to be attenuated in the process of being multiply-reflected bythe first and second reflecting films 40, 50, and is never transmitted.

Therefore, as shown in FIG. 10, by varying the dimension of the firstgap G1 between the first and second reflecting films 40, 50 sequentiallyto g0, g1, g2, and then g3 so as to be narrowed, the light beamtransmitted through the optical filter 10 varies in the wavelength,namely the transmission peak wavelength, sequentially to λ0, λ1, λ2, andthen λ3 (λ0>λ1>λ2>λ3) so as to be shortened.

Here, although the values of L, M, and N can arbitrarily be changed, theintegers satisfying L≧3, M≧3, and N≧6 are preferable. If the integerssatisfying L≧3, M≧3, and N≧6 are used, it is possible to switch theinner electrical potential difference ΔVseg1 and the outer electricalpotential difference ΔVseg2 set respectively to the first and secondsegment electrodes 62, 64 from the first electrical potential differenceΔV1 to the second electrical potential difference ΔV2 larger than thefirst electrical potential difference ΔV1, and then the third electricalpotential difference ΔV3 larger than the second electrical potentialdifference ΔV2.

As shown in FIG. 9, the electrical potential difference control section110 firstly applies the voltages VO1 through VO5 sequentially to thesecond segment electrode 64 located outside. Since the second electrode70 is set to 0V, the electrical potential difference between the secondelectrode 70 and the second segment electrode 64, namely the outerelectrical potential difference ΔVseg2, can sequentially be increased tothe first electrical potential difference VO1, the second electricalpotential difference VO2, the third electrical potential difference VO3,the fourth electrical potential difference VO4, and then the fifthelectrical potential difference VO5. Thus, the dimension of the firstgap G1 between the first and second reflecting films 40, 50 issequentially reduced in such a manner as g0→g1→g2→g3→g4. As a result,the wavelength λ of the light beam transmitted through the opticalfilter 10, namely the transmission peak wavelength, sequentially variesso as to be shortened in such a manner as λ0→λ1→λ2→λ3→λ4.

Subsequently, as shown in FIG. 9, the electrical potential differencecontrol section 110 sequentially applies the voltages VI1 through VI4 tothe first segment electrode 62 located inside while keeping theapplication of the maximum applied voltage VO5 to the second segmentelectrode 64. Since the second electrode 70 is set to 0V, the electricalpotential difference between the second electrode 70 and the firstsegment electrode 62, namely the inner electrical potential differenceΔVseg1, can sequentially be increased to the first electrical potentialdifference VI1, the second electrical potential difference VI2, thethird electrical potential difference VI3, and then the fourthelectrical potential difference VI4. Thus, the dimension of the firstgap G1 between the first and second reflecting films 40, 50 issequentially reduced in such a manner as g5→g6→g7→g8. As a result, thewavelength λ of the light beam transmitted through the optical filter10, namely the transmission peak wavelength, sequentially varies so asto be shortened in such a manner as λ5→λ6→λ7→λ8.

Since the electrical potential difference control section 110 switchesthe outer electrical potential difference ΔVseg2 at least from the firstelectrical potential difference VO1 to the second electrical potentialdifference VO2 larger than the first electrical potential differenceVO1, and further the third electrical potential difference VO3 largerthan the second electrical potential difference VO2, and furtherswitches the inner electrical potential difference ΔVseg1 at least fromthe first electrical potential difference VI1 to the second electricalpotential difference VI2 larger than the first electrical potentialdifference VI1, and further the third electrical potential differenceVI3 larger than the second electrical potential difference VI2, it ispossible to suppress the damped free vibration of the second substrate30, the movable substrate, and thus the prompt wavelength varyingoperation can be performed. Moreover, the electrical potentialdifference control section 110 applies three or more voltage values(voltage of 0 can be included) to each of the first and second segmentelectrodes 62, 64, namely applies at least the first segment voltageVI1, the second segment voltage VI2, and the third segment voltage VI3to the first segment electrode 62, and applies at least the firstsegment voltage VO1, the second segment voltage VO2, and the thirdsegment voltage VO3 to the second segment electrode 64. Therefore, itbecomes possible to vary the gap in three or more levels only by drivingeither one of the first and second segment electrodes 62, 64, andtherefore, it can be prevented to unnecessarily increase the number ofsegment electrodes of the first electrode 60.

1.2.3. Voltage Variation (Absolute Value of Difference Between FirstElectrical Potential Difference and Second Electrical PotentialDifference, Etc.)

The electrical potential difference control section 110 can make theabsolute value of the difference between the second electrical potentialdifference and the third electrical potential difference smaller thanthe absolute value of the difference between the first electricalpotential difference and the second electrical potential difference withrespect to each of the inner electrical potential difference ΔVseg1 andthe outer electrical potential difference ΔVseg2. Since in the presentembodiment the second electrode 70 is fixed to the common voltage of 0V,the absolute value of the difference between the first electricalpotential difference and the second electrical potential difference asthe outer electrical potential difference ΔVseg2, for example, isequivalent to the voltage variation ΔVO1 between the first segmentvoltage VO1 and the second segment voltage VO2 applied to the secondsegment electrode 64 as shown in FIGS. 8 and 9. As shown in FIGS. 8 and9, the voltage variations of the outer electrical potential differenceΔVseg2 are in a descending relationship of ΔVO1>ΔVO2>ΔVO3>ΔVO4, and thevoltage variations of the inner electrical potential difference ΔVseg1are also in a descending relationship of ΔVI1>ΔVI2>ΔVI3.

The reason of setting such a relationship as described above is asfollows. According to Formula 1 described above, the electrostaticattractive force F is proportional to the square of the electricalpotential difference (the applied voltage V to the first electrode 60 inthe present embodiment) between the first and second electrodes 60, 70.FIG. 11 is a graph (showing F=V²) of the electrostatic attractive forceF proportional to the square of the electrical potential difference V.As shown in FIG. 11, when switching the electrical potential differenceV in the ascending direction to the first electrical potentialdifference, the second electrical potential difference, and then thethird electrical potential difference, if the absolute value ΔV1 of thedifference between the first electrical potential difference and thesecond electrical potential difference and the absolute value ΔV2 of thedifference between the second electrical potential difference and thethird electrical potential difference are the same (ΔV1=ΔV2 in FIG. 11),it results that the increment ΔF of the electrostatic attractive forcerapidly increases from ΔF1 to ΔF2, which causes an overshoot.

Therefore, it is arranged that the absolute value ΔV2 of the differencebetween the second electrical potential difference and the thirdelectrical potential difference is smaller than the absolute value ΔV1of the difference between the first electrical potential difference andthe second electrical potential difference. Thus, it is possible tosuppress the rapid increase in the electrostatic attractive force whenthe gap is narrowed to thereby further suppress the overshoot, and thus,the prompter wavelength variation operation can be realized.

1.2.4. Voltage Application Period

The electrical potential difference control section 110 can set theperiod during which the electrical potential difference is set to thesecond electrical potential difference longer than the period duringwhich the electrical potential difference is set to the first electricalpotential difference, and the period during which the electricalpotential difference is set to the third electrical potential differencelonger than the period during which the electrical potential differenceis set to the second electrical potential difference with respect toeach of the inner electrical potential difference ΔVseg1 and the outerelectrical potential difference ΔVseg2. In the present embodiment, asshown in FIG. 9, regarding the outer electrical potential differenceΔVseg2, the period TO2 of the second electrical potential difference VO2is longer than the period TO1 of the first electrical potentialdifference VO1, the period TO3 of the third electrical potentialdifference VO3 is longer than the period TO2 of the second electricalpotential difference VO2, and the periods are in an ascendingrelationship of TO1<TO2<TO3<TO4<TO5. Similarly, as shown in FIG. 9,regarding the inner electrical potential difference ΔVseg1, the periodTI2 of the second electrical potential difference VI2 is longer than theperiod TI1 of the first electrical potential difference VI1, the periodTI3 of the third electrical potential difference VI3 is longer than theperiod TI2 of the second electrical potential difference VI2, and theperiods are in an ascending relationship of TI1<TI2<TI3<TI4.

When the second electrical potential difference larger than the firstelectrical potential difference is set, or the third electricalpotential difference larger than the second electrical potentialdifference is set, the restoring force of the second substrate 30 alsoincreases. Therefore, the time until the second substrate 30 stopsbecomes longer. In other words, the time until the first gap G1 betweenthe first and second reflecting films 40, 50 settles in place becomeslonger. In contrast, by setting the period set for the second electricalpotential difference longer than the period set for the first electricalpotential difference, and setting the period set for the thirdelectrical potential difference longer than the period set for thesecond electrical potential difference as in the present embodiment, itis possible to settle the first gap G1 at a predetermined value.

1.2.5. First Specific Example of Electrical Potential Difference, Gap,and Variable Wavelength

FIG. 12 is a characteristics table showing data of a first specificexample regarding the electrical potential difference, the gap, and thevariable wavelength shown in FIG. 8. The data numbers 1 through 9 inFIG. 12 correspond to the data numbers 1 through 9 in FIG. 8. FIG. 13 isa graph showing a relationship between the applied voltage and the gapshown in FIG. 12. FIG. 14 is a graph showing a relationship between theapplied voltage and the transmission peak wavelength shown in FIG. 12.

In FIG. 12, in order for making the transmission peak wavelengthvariable in 9 levels from the maximum wavelength λ0 (=700 nm) to theminimum wavelength λ8 (=380 nm) of the transmission peak wavelength, thefirst gap G1 between the first and second reflecting films 40, 50 ismade variable in 9 levels from the maximum gap g0 (=300 nm) to theminimum gap g8 (=140 nm) (see also FIG. 13). In accordance therewith,the transmission peak wavelength is made variable in 9 levels from themaximum wavelength λ0 to the minimum wavelength λ8 (see also FIG. 14).Moreover, in FIG. 12, by setting the 9 levels of gaps g0 through g8 fromthe maximum gap g0 to the minimum gap g8 at regular intervals (=20 nm),the 9 levels of the wavelength λ0 through λ8 from the maximum wavelengthλ0 to the minimum wavelength λ8 are also set to have regular intervals(=40 nm). By varying the dimension of the first gap G1 between the firstand second reflecting films so as to sequentially decrease by a constantamount, the transmission peak wavelength is also shortened by a constantvalue.

The electrical potential difference control section 110 sets the outerelectrical potential difference ΔVseg2 sequentially to VO1 (=16.9V), VO2(=21.4V), VO3 (=25V), VO4 (=27.6V), and then VO5 (=29.8V), and then setsthe inner electrical potential difference ΔVseg1 sequentially to VI1(=16.4V), VI2 (=22.2V), VI3 (=26.3V), and then VI4 (=29.3V) whilekeeping the outer electrical potential difference ΔVseg2 at VO5(=29.8V).

It should be noted that the dimension of the first gap G1 between thefirst and second reflecting films 40, 50 is more significantly affectedby the electrostatic attractive force F1 based on the inner electricalpotential difference ΔVseg1 than the electrostatic attractive force F2based on the outer electrical potential difference ΔVseg2. Therefore, ifthe inner electrical potential difference ΔVseg1 is firstly varied, andthen the outer electrical potential difference ΔVseg2 is varied whilekeeping the inner electrical potential difference ΔVseg1 at a constantvalue, since the electrostatic attractive force F1 by the innerelectrical potential difference ΔVseg1 is dominant, the gap between thefirst and second reflecting films 40, 50 does not vary so largely as theouter electrical potential difference ΔVseg2 varies. Therefore, in thepresent embodiment, the outer electrical potential difference ΔVseg2 isvaried first, and then the inner electrical potential difference ΔVseg1is varied while keeping the outer electrical potential difference ΔVseg2at a constant value.

After the outer electrical potential difference ΔVseg2 reaches the outermaximum electrical potential difference VO5, the electrical potentialdifference control section 110 varies the inner electrical potentialdifference ΔVseg1 while keeping the outer electrical potentialdifference ΔVseg2 at the outer maximum electrical potential differenceVO5. According to this process, a further gap variation corresponding toone step from the first gap G1 set by the outer maximum electricalpotential difference VO5 becomes possible due to the application of theinner electrical potential difference ΔVseg1. Moreover, since the outermaximum electrical potential difference VO5 has already been reached, itis not required to further vary the outer electrical potentialdifference ΔVseg2 after the inner electrical potential difference ΔVseg1is applied. Therefore, when varying the outer electrical potentialdifference ΔVseg2, no harmful influence is caused by the dominantelectrostatic attractive force F1 based on the inner electricalpotential difference ΔVseg1.

When the electrical potential difference control section 110 set theinner electrical potential difference ΔVseg1 to the inner maximumelectrical potential difference VI4, the first gap G1 between the firstand second reflecting films 40, 50 is set to the minimum distance g8.The outer maximum electrical potential difference VO5 and the innermaximum electrical potential difference VI4 can be set substantiallyequal to each other within a range not exceeding the maximum supplyvoltage Vmax supplied to the electrical potential difference controlsection 110. In the present embodiment the maximum supply voltage Vmax(=30V), for example, is supplied to the electrical potential differencecontrol section 110 from the power supply 120 shown in FIG. 7. On thisoccasion, the outer maximum electrical potential difference VO5 is setto 29.8V not exceeding the maximum supply voltage Vmax (30V), andfurther the inner maximum electrical potential difference VI4 is alsoset to 29.3V not exceeding the maximum supply voltage Vmax (30V).

In FIG. 12, although a minute difference of 0.5V exists between theouter maximum electrical potential difference VO5 and the inner maximumelectrical potential difference VI4, it can be said that they aresubstantially the same. The minute difference occurs as a result of thedesign made under the intention that the transmission peak wavelength isobtained using the full scale (see FIGS. 13 and 14) of the range notexceeding the maximum supply voltage Vmax (30V) with respect to each ofthe inner electrical potential difference ΔVseg1 and the outerelectrical potential difference ΔVseg2. It is possible to strictlyconform the outer maximum electrical potential difference VO5 and theinner maximum electrical potential difference VI4 to each other byadjusting the area ratio between the first and second segment electrodes62, 64 and so on. However, there is little necessity for strictlyconforming them. It should be noted that according to the drive methodof the present embodiment as explained with reference to FIG. 5A, bymaking the outer maximum electrical potential difference VO5 and theinner maximum electrical potential difference VI4 substantially equal toeach other, there is obtained an advantage that even electrostaticattractive force can be generated in almost entire circumference(including the opposed area 74A1 to the first slit 64C) of the fourthsegment electrode 74 located outside.

In the present embodiment the electrical potential difference controlsection 110 sequentially applies the voltages to K (=2) electrodes,namely the first and second segment electrodes 62, 64 to thereby makethe first gap G1 between the first and second reflecting films 40, 50variable in N (=9) levels in total. On this occasion, the minimum valueof the voltage variation between the applied voltages to be applied tothe same segment electrode 62 (or 64) out of the K (=2) electrodes,namely the first and second segment electrodes 62, 64 is defined asΔVkmin. In the example shown in FIGS. 8 and 12, regarding the firstsegment electrode 62, ΔVkmin=ΔVI3=3.0V is obtained, and regarding thesecond segment electrode 64, ΔVkmin=ΔVO4=2.2V is obtained. Consideringthe fact that the power supply noise is about 0.1V, it is obvious fromthe comparison with the comparative example described below that theminimum voltage value ΔVkmin has low sensitivity to noise.

1.2.6. Second Specific Example of Electrical Potential Difference, Gap,and Variable Wavelength

In the second specific example, as shown in FIGS. 15A and 15B, the firstelectrode 61 shown in FIG. 15A is used instead of the first electrode 60of the first specific example, and the second electrode 71 shown in FIG.15B is used instead of the second electrode 70 of the first specificexample. Specifically, the first and second electrodes 61, 71 of thesecond specific example are not divided into segments.

FIG. 16 is a characteristics table showing the data of the electricalpotential difference between the first and second electrodes 61, 71shown in FIGS. 15A and 15B, and the gap and the variable wavelengthobtained therefrom. The data numbers 1 through 9 in FIG. 16 correspondto the data numbers 1 through 9 in FIGS. 8 and 12. FIG. 17 is a graphshowing a relationship between the applied voltage and the gap shown inFIG. 16. FIG. 18 is a graph showing a relationship between the appliedvoltage and the transmission peak wavelength shown in FIG. 16.

Also in FIG. 16, in order for making the transmission peak wavelengthvariable in 9 levels from the maximum wavelength λ0 (=700 nm) to theminimum wavelength λ8 (=380 nm) of the transmission peak wavelength, thefirst gap G1 between the first and second reflecting films 40, 50 ismade variable in 9 levels from the maximum gap g0 (=300 nm) to theminimum gap g8 (=140 nm) (see also FIG. 16). In accordance therewith,the transmission peak wavelength is made variable in 9 levels from themaximum wavelength λ0 to the minimum wavelength λ8 (see also FIG. 17).

It should be noted that in the second specific example the 9 levels ofvoltage applied to the first electrode 61 as a unique electrode are setwithin the full scale with the maximum supply voltage Vmax (30V).

The minimum voltage variation between the 9 levels of applied voltage inthe case of forming the first electrode 61 of a unique electrode as inthe second specific example is defined as ΔV1min. In the example shownin FIG. 16, ΔV1min=0.9V is provided. Considering the fact that the powersupply noise is about 0.1V, the minimum voltage variation ΔV1min of thesecond specific example has high sensitivity to noise.

In comparison between the minimum voltage variation ΔVkmin of the firstspecific example and the minimum voltage variation ΔV1min of the secondspecific example, ΔV1min<ΔVkmin becomes true, and therefore, accordingto the first specific example, the sensitivity to noise can be reduced.

2. Modified Example of Optical Filter

FIG. 19 shows an optical filter 11 that is different from the opticalfilter 10 shown in FIG. 1. In a first substrate 21 shown in FIG. 19, thesecond surface 20A2 provided with the first electrode 60 in FIG. 1includes a “2-1” surface 20A21 in the periphery of the first surface20A1 provided with the first reflecting film 40 in a plan view, and a“2-2” surface 20A22 disposed in the periphery of the 2-1 surface 20A21in the plan view and having a step 24 with the 2-1 surface 20A21.

The first segment electrode 62 is disposed on the 2-1 surface 20A21, thesecond segment electrode 64 is disposed on the 2-2 surface 20A22, and aninitial value of a gap G22 between the second segment electrode 64 andthe second electrode 70 and an initial value of a gap G21 between thefirst segment electrode 62 and the second electrode 70 are differentfrom each other.

The reason of setting such a relationship as described above is asfollows. Among the gaps G21, G22 in the initial state, the gap G22 inthe initial state, which is driven first, and corresponds to the secondsegment electrode 64, for example, is narrowed by the electrostaticattractive force acting between the second segment electrode 64 and thesecond electrode 70. On this occasion, the gap G21 is also narrowed atthe same time to be smaller than the initial gap. Therefore, whenstarting to drive the first segment electrode 62, the gap G21 is smallerthan the initial value.

Here, it is assumed that the 2-1 surface 20A21 and the 2-2 surface 20A22are coplanar with each other, and the initial values of the gaps G21,G22 are the same. In this case, the gap G22 in the case of first drivingthe second segment electrode 64, for example, is larger than the gap G21in the case of driving the first segment electrode 62 later. Therefore,it becomes necessary to set the electrostatic attractive force in thecase of first driving the second segment electrode 64 to be excessivelystronger than the electrostatic attractive force in the case in whichthe first segment electrode 62 is driven.

Therefore, in this case, it is preferable that to set the initial valueof the gap G22 to be smaller than the initial value of the gap G21 asshown in FIG. 19. It should be noted that in the case of driving thefirst segment electrode 62 first, it is sufficient to set the initialvalue of the gap G21 to be smaller than the initial value of the gapG22.

3. Analytical Instrument

FIG. 20 is a block diagram showing a schematic configuration of acolorimeter as an example of an analytical instrument according to anembodiment of the invention.

In FIG. 20, the colorimeter 200 is provided with a light source device202, a spectral measurement device 203, and a colorimetric controldevice 204. The colorimeter 200 emits, for example, a white light beamfrom the light source device 202 toward the test object A, and theninput the test target light beam, the light beam reflected by the testobject A, to the spectral measurement device 203. Subsequently, thecolorimeter 200 disperses the test target light beam with the spectralmeasurement device 203, and then spectral characteristics measurementfor measuring the intensity of each of the light beams with respectivewavelengths obtained by the dispersion is performed. In other words, thecolorimeter 200 makes the test target light beam as the light beamreflected by the test object A enter the optical filter (an etalon) 10,and then performs the spectral characteristics measurement for measuringthe intensity of the light beam transmitted through the etalon 10.Subsequently, the colorimetric control device 204 performs thecolorimetric process of the test object A, namely analyzes thewavelengths of the colored light beams included therein, and theproportions of the colored light beams, based on the spectralcharacteristics thus obtained.

The light source device 202 is provided with a light source 210 and aplurality of lenses 212 (one of the lenses is shown in FIG. 20), andemits a white light beam to the test object A. Further, the plurality oflenses 212 includes a collimator lens, and the light source device 202modifies the white light beam emitted from the light source 210 into aparallel light beam with the collimator lens, and emits it from theprojection lens not shown to the test object A.

As shown in FIG. 20, the spectral measurement device 203 is providedwith the etalon 10, a light receiving section 220 as the light receivingsection, a drive circuit 230, and a control circuit section 240.Further, the spectral measurement device 203 has an entrance opticallens not shown disposed at a position opposed to the etalon 10, theentrance optical lens guiding the reflected light beam (the test targetlight beam) reflected by the test object A into the inside thereof.

The light receiving section 220 is composed of a plurality ofphotoelectric conversion elements, and generates an electric signalcorresponding to the received light intensity. Further, the lightreceiving section 220 is connected to the control circuit section 240,and outputs the electric signal thus generated to the control circuitsection 240 as a light reception signal.

The drive circuit 230 is connected to the first electrode 60 and thesecond electrode 70 of the etalon 10, and the control circuit section240. The drive circuit 230 applies the drive voltage between the firstelectrode 60 and the second electrode 70 based on the drive controlsignal input from the control circuit section 240 to thereby displacethe second substrate 30 to a predetermined displacement position. Thedrive voltage can be applied so that the desired electrical potentialdifference is caused between the first electrode 60 and the secondelectrode 70, and for example, it is also possible to apply apredetermined voltage to the first electrode 60 while setting the secondelectrode 70 to the ground potential. A direct-current voltage ispreferably used as the drive voltage.

The control circuit section 240 controls overall operations of thespectral measurement device 203. As shown in FIG. 20, the controlcircuit section 240 is mainly composed of, for example, a CPU 250 and astorage section 260. Further, the CPU 250 performs a spectralmeasurement process based on various programs and various data stored inthe storage section 260. The storage section 260 is configured includinga recording medium such as a memory or a hard disk drive, and stores thevarious programs and various data so as to be arbitrarily retrieved.

Here, the storage section 260 stores a voltage adjustment section 261, agap measurement section 262, a light intensity recognition section 263,and a measurement section 264 as a program. It should be noted that asdescribed above the gap measurement section 262 can be omitted.

Further, the storage section 260 stores voltage table data 265 shown inFIG. 8 containing voltage values to be applied to the electrostaticactuators 80, 90 for controlling the spacing of the first gap G1 and thetime periods, during which the respective voltage values are applied, inconjunction with each other.

The colorimetric control device 204 is connected to the spectralmeasurement device 203 and the light source device 202, and performs thecontrol of the light source device 202 and the colorimetric processbased on the spectral characteristics obtained by the spectralmeasurement device 203. As the colorimetric control device 204, ageneral-purpose personal computer, a handheld terminal, acolorimetric-dedicated computer, and so on can be used.

Further, as shown in FIG. 20, the colorimetric control device 204 isconfigured including a light source control section 272, a spectralcharacteristics obtaining section 270, a colorimetric processing section271, and so on.

The light source control section 272 is connected to the light sourcedevice 202. Further, the light source control section 272 outputs apredetermined control signal to the light source device 202 based on,for example, a setting input by the user to thereby make the lightsource device 202 emit a white light beam with a predeterminedbrightness.

The spectral characteristic obtaining section 270 is connected to thespectral measurement device 203, and obtains the spectralcharacteristics input from the spectral measurement device 203.

The colorimetric processing section 271 performs the colorimetricprocess for measuring the chromaticity of the test object A based on thespectral characteristics. For example, the colorimetric processingsection 271 performs a process of making a graph of the spectralcharacteristics obtained from the spectral measurement device 203, andthen outputting it to an output device such as a printer or a displaynot shown.

FIG. 21 is a flowchart showing the spectral measurement operation of thespectral measurement device 203. Firstly, the CPU 250 of the controlcircuit section 240 starts the voltage adjustment section 261, the lightintensity recognition section 263, and the measurement section 264.Further, the CPU 250 initializes a measurement count variable “n” (setn=0) as an initial state (step S1). It should be noted that themeasurement count variable n takes an integer value equal to or largerthan 0.

Subsequently, the measurement section 264 measures (step S2) theintensity of the light beam transmitted through the etalon 10 in theinitial state, namely the state in which no voltage is applied to theelectrostatic actuators 80, 90. It should be noted that it is alsopossible to previously measure the dimension of the first gap G1 in theinitial state, for example, at the time of manufacturing of the spectralmeasurement device and store it in the storage section 260. Then, themeasurement section 264 outputs the intensity of the transmitted lightbeam and the dimension of the first gap G1 in the initial state obtainedhere to the colorimetric control device 204.

Subsequently, the voltage adjustment section 261 retrieves (step S3) thevoltage table data 265 stored in the storage section 260. Further, thevoltage adjustment section 261 adds (step S4) “1” to the measurementcount variable n.

Subsequently, the voltage adjustment section 261 obtains (step S5) thevoltage data of the first and second segment electrodes 62, 64 and thevoltage application period data corresponding to the measurement countvariable n from the voltage table data 265. Then, the voltage adjustmentsection 261 outputs the drive control signal to the drive circuit 230 tothereby perform (step S6) the process of driving the electrostaticactuators 80, 90 in accordance with the data of the voltage table data265.

Further, the measurement section 264 performs (step S7) the spectralmeasurement process at the application time elapse timing. Specifically,the measurement section 264 makes the light intensity recognitionsection 263 measure the intensity of the transmitted light. Further, themeasurement section 264 performs the control of outputting the spectralmeasurement result, which includes the intensity of the transmittedlight beam thus measured and the wavelength of the transmitted lightbeam in conjunction with each other, to the colorimetric control device204. It should be noted that in the measurement of the light intensity,it is also possible to store the data of the light intensity of aplurality of times of measurement or all of the times of the measurementin the storage section 260, and then measure the light intensity of eachof the turns of the measurement in a lump after the data of the lightintensity of a plurality of times of measurement or all of the data ofthe light intensity has been obtained.

Subsequently, the CPU 250 determines (step S8) whether or not themeasurement count variable n reaches the maximum value N, and if itdetermines that the measurement count variable n is equal to N, itterminates the series of spectral measurement operation. In contrast, ifit is determined in the step S8 that the measurement count variable n issmaller than N, the CPU 250 returns to the step S4 and performs theprocess of adding “1” to the measurement count variable n, and thenrepeats the process of the steps S5 through S8.

4. Optical Apparatus

FIG. 22 is a block diagram showing a schematic configuration of atransmitter of a wavelength division multiplexing system as an exampleof an optical apparatus according to an embodiment of the invention. Inthe wavelength division multiplexing (WDM) communication, using theproperty of the light that the signals with respective wavelengthsdifferent from each other do not interfere each other, by using aplurality of light signals with respective wavelengths different fromeach other in a single optical fiber in a multiplexed manner, it becomespossible to increase the data transmission quantity without expandingthe optical fiber lines.

In FIG. 22, a wavelength division multiplexing transmitter 300 has anoptical filter 10 to which a light beam from a light source 301 isinput, and a plurality of light beams with respective wavelengths λ0,λ1, λ2, . . . is transmitted through the optical filter 10. Transmissiondevices 311, 312, and 313 are provided corresponding to the respectivewavelengths. Optical pulse signals corresponding to a plurality ofchannels output from the transmission devices 311, 312, and 313 arecombined by a wavelength division multiplexing device 321 into onesignal, and then output to an optical fiber transmission channel 331.

The invention can also be applied to an optical code divisionmultiplexing (OCDM) transmitter in a similar manner. This is becausealthough in the OCDM the channels are discriminated by pattern matchingof encoded optical pulse signals, the optical pulses constituting theoptical pulse signals include light components with respectivewavelengths different from each other.

Although some embodiments are hereinabove explained, it should beunderstood by those skilled in the art that various modifications notsubstantially departing from the novel matters and the effects of theinvention are possible. Therefore, such modified examples should beincluded in the scope of the invention. For example, a term described atleast once with a different term having a broader sense or the samemeaning in the specification or the accompanying drawings can bereplaced with the different term in any part of the specification or theaccompanying drawings.

The invention is not limited to the configuration provided with the steponly in the first substrate 20, but rather it is possible to provide thestep to at least one of the first and second substrates 20, 30. Forexample, although in FIG. 1 it is assumed that the first substrate 20provided with the step is the fixed substrate, it is also possible toadopt a configuration in which the first substrate 20 is the movablesubstrate. FIG. 23 shows another embodiment of the invention providing astep 38 to the second substrate 30 as the movable substrate shown inFIG. 1.

In FIG. 23, the second opposed surface 30A of the second substrate 30opposed to the first substrate 20 includes the first surface 30A1provided with the second reflecting film 50 and the second surface 30A2disposed in the periphery of the first surface 30A1 in the plan view,and provided with the second electrode 70. The first surface 30A1 andthe second surface 30A2 are not coplanar with each other, there is thestep 38 between the first surface 30A1 and the second surface 30A2, andthe first surface 30A1 is placed nearer to the first substrate 20 thanthe second surface 30A2. Thus, the relationship of (first gapG1)<(second gap G2) becomes true with initial values in the non-voltageapplication state.

Here, in FIG. 23, if the surface on the opposite side of the secondsubstrate 30 to the second opposed surface 30A is formed as a flatsurface, the area provided with the second reflecting film 50 can bemade as the thick-wall section 32. In such a manner as described above,the second substrate 30 can be made movable while maintaining theparallelism of the second reflecting film 50.

It should be noted that as described above one of the pair of opposedsubstrates 20, 30, which is provided with a step, can be called thefirst substrate, and in FIG. 23, the second substrate 30, the secondreflecting film 50, and the second electrode 70 can also be called thefirst substrate, the first reflecting film, and the first electrode,respectively. Further, in the embodiment shown in FIG. 23, it is alsopossible to provide the step corresponding to the step 24 shown in FIG.19 to the second substrate 30, or to both of the first and secondsubstrates 20, 30. On this occasion, it is preferable to also providethe step to the opposite surface of the second substrate 30 to thesecond opposed surface 30A so as to form the area provided with thesecond electrode 70 as the thin-wall section 34.

FIG. 24 shows still another embodiment of the invention in which thesteps 22, 38 shown in FIGS. 1 and 23 are provided respectively to thefirst and second substrates 20, 30. According also to thisconfiguration, the relationship of (first gap G1)<(second gap G2)becomes true with initial values in the non-voltage application state.Also in the embodiment shown in FIG. 24, it is also possible to providethe step corresponding to the step 24 shown in FIG. 19 to the secondsubstrate 30, or to both of the first and second substrates 20, 30.

1. An optical filter comprising: a first substrate; a second substrateopposed to the first substrate; a first reflecting section disposed on afirst opposed surface of the first substrate, the first opposed surfacebeing opposed to the second substrate; a second reflecting sectiondisposed on a second opposed surface of the second substrate, the secondopposed surface being opposed to the first substrate, and the secondreflecting section being opposed to the first reflecting section; afirst electrode disposed on the first opposed surface of the firstsubstrate at a peripheral position relative to the first reflectingsection in a plan view; and a second electrode disposed on the secondopposed surface of the second substrate, and opposed to the firstelectrode, wherein at least one of the first opposed surface and thesecond opposed surface includes a step section, and in a state that novoltage is applied between the first electrode and the second electrode,a first gap between the first reflecting section and the secondreflecting section is smaller than a second gap between the firstelectrode and the second electrode.
 2. The optical filter according toclaim 1, wherein the first opposed surface of the first substrateincludes a first surface and a second surface disposed in a periphery ofthe first surface in the plan view, the second surface forming a stepwith the first surface, the first surface is provided with the firstreflecting section, and the second surface is provided with the firstelectrode.
 3. The optical filter according to claim 1, wherein the firstopposed surface of the first substrate includes a first surface and asecond surface disposed in a periphery of the first surface in the planview, the second surface forming a first step with the first surface,the first surface is provided with the first reflecting section, thesecond surface is provided with the first electrode, the second opposedsurface of the second substrate includes a third surface and a fourthsurface disposed in a periphery of the third surface in the plan view,the fourth surface forming a second step with the third surface, thethird surface is provided with the second reflecting section, and thefourth surface is provided with the second electrode.
 4. The opticalfilter according to claim 2, wherein the second substrate is movablysupported with respect to the first substrate, and the second substratehas a first thickness where the second reflecting section is disposedthat is larger than a second thickness of the second substrate where thesecond electrode is disposed.
 5. The optical filter according to claim1, wherein the first electrode is divided into K (K is an integer≧2)segment electrodes electrically isolated from each other, and the secondelectrode is a common electrode having the same electrical potential. 6.The optical filter according to claim 2, wherein the first electrode isdivided into K (K is an integer≧2) segment electrodes electricallyisolated from each other, the second electrode is a common electrodehaving the same electrical potential, the K segment electrodes includeat least first and second segment electrodes respectively includingring-like electrode sections disposed so as to have a concentric ringshape with respect to a center of the first reflecting section, and thefirst segment electrode is disposed on an inner circumferential side ofthe second segment electrode.
 7. The optical filter according to claim6, wherein a first lead wire is connected to the first segmentelectrode, the second segment electrode is provided with a first slitmaking the second segment electrode discontinuous, and the first leadwire extends outside the second segment electrode via the first slit. 8.The optical filter according to claim 7, wherein the second electrodeprovided to the second substrate has third and fourth segment electrodesrespectively including ring-like electrode sections disposed so as tohave a concentric ring shape with respect to a center of the secondreflecting section, the third segment electrode is opposed to the firstsegment electrode, the fourth segment electrode is opposed to the secondsegment electrode, and the third and fourth segment electrodes areelectrically connected to each other.
 9. The optical filter according toclaim 8, wherein the fourth segment electrode is formed continuously ata position opposed to the first slit.
 10. The optical filter accordingto claim 8, wherein the fourth segment electrode is provided with asecond slit making the fourth segment electrode discontinuous at aposition opposed to the first slit.
 11. The optical filter according toclaim 7, wherein at least one of the first and second substrates isformed as a rectangular substrate, the first lead wire extends in afirst direction along a first diagonal of the at least one of the firstand second substrates from the first segment electrode, a second leadwire extending in a second direction opposite to the first direction onthe first diagonal is connected to the second segment electrode, a thirdlead wire extending in a third direction along a second diagonal of theat least one of the first and second substrates is connected to thethird and fourth segment electrodes so as to connect the third andfourth segment electrodes to each other, a fourth lead wire extending ina fourth direction opposite to the third direction on the seconddiagonal is connected to the third and fourth segment electrodes so asto connect the third and fourth segment electrodes to each other, andfirst through fourth connection electrode sections to which the firstthrough fourth lead wires are respectively connected are disposed atfour corners of the at least one of the first and second substrates inthe plan view.
 12. The optical filter according to claim 6, wherein thefirst segment electrode has a first ring width, the second segmentelectrode has a second ring width, and the second ring width is greaterthan the first ring width.
 13. The optical filter according to claim 6,wherein the second surface of the first substrate includes a 2-1surface, and a 2-2 surface disposed in a periphery of the 2-1 surface inthe plan view, the 2-2 surface forming a step with the 2-1 surface, thefirst segment electrode is disposed on the 2-1 surface, the secondsegment electrode is disposed on the 2-2 surface, and a first spacebetween the second segment electrode and the second electrode when novoltage is applied between the second segment electrode and the secondelectrode is different from a second space between the first segmentelectrode and the second electrode when no voltage is applied betweenthe first segment electrode and the second electrode.
 14. The opticalfilter according to claim 13, wherein the first space between the secondsegment electrode and the second electrode is smaller than the secondspace between the first segment electrode and the second electrode whenno voltage is applied between the first segment electrode and the secondelectrode.
 15. The optical filter according to claim 5, furthercomprising: an electrical potential difference control section adaptedto control an electrical potential difference between each of the Ksegment electrodes and the second electrode, wherein the electricalpotential difference control section applies a plurality of voltagevalues set for each of the K segment electrodes to each of the K segmentelectrodes to switch each of the electrical potential differences from afirst electrical potential difference to a second electrical potentialdifference that is larger than the first electrical potentialdifference, and then to a third electrical potential difference that islarger than the second electrical potential difference, the firstthrough third electrical potential differences being set for each of theK segment electrodes.
 16. The optical filter according to claim 6,further comprising: an electrical potential difference control sectionadapted to respectively control an inner electrical potential differencebetween the first segment electrode and the second electrode and anouter electrical potential difference between the second segmentelectrode and the second electrode, wherein the electrical potentialdifference control section applies a plurality of voltage values set foreach of the first and second segment electrodes to each of the first andsecond segment electrodes to switch each of the inside and outerelectrical potential differences from a first electrical potentialdifference to a second electrical potential difference that is largerthan the first electrical potential difference, and then to a thirdelectrical potential difference that is larger than the secondelectrical potential difference, the first through third electricalpotential differences being set for each of the first and second segmentelectrodes.
 17. The optical filter according to claim 16, wherein theelectrical potential difference control section applies: a first segmentvoltage to the first segment electrode to set the inner electricalpotential difference to the first electrical potential difference, asecond segment voltage to the first segment electrode to set the innerelectrical potential difference to the second electrical potentialdifference, a third segment voltage to the first segment electrode toset the inner electrical potential difference to the third electricalpotential difference, a fourth segment voltage to the second segmentelectrode to set the outer electrical potential difference to the firstelectrical potential difference, a fifth segment voltage to the secondsegment electrode to set the outer electrical potential difference tothe second electrical potential difference, and a sixth segment voltageto the second segment electrode to set the outer electrical potentialdifference to the third electrical potential difference.
 18. The opticalfilter according to claim 16, wherein with respect to each of the innerelectrical potential difference and the outer electrical potentialdifference, an absolute value of a difference between the secondelectrical potential difference and the third electrical potentialdifference is smaller than an absolute value of a difference between thefirst electrical potential difference and the second electricalpotential difference.
 19. The optical filter according to claim 16,wherein with respect to each of the inner electrical potentialdifference and the outer electrical potential difference, a periodduring which the electrical potential difference is set to the secondelectrical potential difference is longer than a period during which theelectrical potential difference is set to the first electrical potentialdifference, and a period during which the electrical potentialdifference is set to the third electrical potential difference is longerthan a period during which the electrical potential difference is set tothe second electrical potential difference.
 20. The optical filteraccording to claim 16, wherein the first gap between the firstreflecting section and the second reflecting section is set to a firstdistance when the electrical potential difference control section setsthe outer electrical potential difference to the first electricalpotential difference, the first gap between the first reflecting sectionand the second reflecting section is set to a second distance that issmaller than the first distance when the electrical potential differencecontrol section sets the outer electrical potential difference to thesecond electrical potential difference, the first gap between the firstreflecting section and the second reflecting section is set to a thirddistance that is smaller than the second distance when the electricalpotential difference control section sets the outer electrical potentialdifference to the third electrical potential difference, and an absolutevalue of a difference between the first distance and the second distanceis substantially equal to an absolute value of a difference between thesecond distance and the third distance.
 21. The optical filter accordingto claim 20, wherein the electrical potential difference control sectionchanges the inner electrical potential difference while keeping theouter electrical potential difference at the third electrical potentialdifference, the first gap between the first reflecting section and thesecond reflecting section is set to a fourth distance when theelectrical potential difference control section sets the innerelectrical potential difference to the first electrical potentialdifference, the first gap between the first reflecting section and thesecond reflecting section is set to a fifth distance that is smallerthan the fourth distance when the electrical potential differencecontrol section sets the inner electrical potential difference to thesecond electrical potential difference, the first gap between the firstreflecting section and the second reflecting section is set to a sixthdistance that is smaller than the fifth distance when the electricalpotential difference control section sets the inner electrical potentialdifference to the third electrical potential difference, and an absolutevalue of a difference between the fourth distance and the fifth distanceis substantially equal to an absolute value of a difference between thefifth distance and the sixth distance.
 22. The optical filter accordingto claim 18, wherein the electrical potential difference control sectionchanges the inner electrical potential difference after the thirdelectrical potential difference reaches a maximum outer electricalpotential difference, while keeping the outer electrical potentialdifference at the maximum outer electrical potential difference.
 23. Theoptical filter according to claim 22, wherein the first gap between thefirst reflecting section and the second reflecting section is set to aminimum distance when the electrical potential difference controlsection sets the third electrical potential difference to a maximuminner electrical potential difference, and the maximum outer electricalpotential difference and the maximum inner electrical potentialdifference are substantially equal to each other within a range of amaximum supply voltage supplied to the electrical potential differencecontrol section.
 24. The optical filter according to claim 15, whereinthe electrical potential difference control section sequentially appliesvoltages to the respective K segment electrodes to vary the first gapbetween the first reflecting section and the second reflecting sectionin N levels, and ΔV1min<ΔVkmin, wherein ΔVkmin is a minimum value of avoltage difference between applied voltages to be applied to the samesegment electrode out of the K segment electrodes and ΔV1min is aminimum voltage variation between N levels of applied voltages assumingthe first electrode is a single electrode.
 25. An analytical instrumentcomprising the optical filter according to claim
 1. 26. An opticalapparatus comprising the optical filter according to claim 1.